Fix a race condition in mount lookups

[stevendpclark]

Description

When adding the new method MatchingMountAndEntry within #28752, no locks were being acquired which lead to some race conditions on mount creations. This has not made it into any released versions of Vault.

WARNING: DATA RACE
Write at 0x00c000b493f8 by goroutine 4501:
  github.com/armon/go-radix.(*node).addEdge()
      /home/runner/go/pkg/mod/github.com/armon/go-radix@v1.0.0/radix.go:43 +0x3ce
  github.com/armon/go-radix.(*Tree).Insert()
      /home/runner/go/pkg/mod/github.com/armon/go-radix@v1.0.0/radix.go:184 +0x2fd
  github.com/hashicorp/vault/vault.(*Router).Mount()
      /home/runner/work/vault/vault/vault/router.go:239 +0x8b4
  github.com/hashicorp/vault/vault.(*Core).enableCredentialInternal()
      /home/runner/work/vault/vault/vault/auth.go:220 +0x1e26
  github.com/hashicorp/vault/vault.(*Core).enableCredential()
      /home/runner/work/vault/vault/vault/auth.go:69 +0x24cb
  github.com/hashicorp/vault/vault.(*SystemBackend).handleEnableAuth()
      /home/runner/work/vault/vault/vault/logical_system.go:3384 +0x2491
  github.com/hashicorp/vault/vault.(*SystemBackend).handleEnableAuth-fm()

...

Previous read at 0x00c000b493f8 by goroutine 4691:
  github.com/armon/go-radix.(*node).getEdge()
      /home/runner/go/pkg/mod/github.com/armon/go-radix@v1.0.0/radix.go:60 +0x3c
  github.com/armon/go-radix.(*Tree).LongestPrefix()
      /home/runner/go/pkg/mod/github.com/armon/go-radix@v1.0.0/radix.go:395 +0xd5
  github.com/hashicorp/vault/vault.(*Router).matchingMountInternal()
      /home/runner/work/vault/vault/vault/router.go:393 +0xe6
  github.com/hashicorp/vault/vault.(*Router).MatchingMountAndEntry()
      /home/runner/work/vault/vault/vault/router.go:383 +0x131
  github.com/hashicorp/vault/vault.(*Core).handleCancelableRequest()
      /home/runner/work/vault/vault/vault/request_handling.go:632 +0x72

TODO only if you're a HashiCorp employee

• Backport Labels: If this PR is in the ENT repo and needs to be backported, backport
  to N, N-1, and N-2, using the backport/ent/x.x.x+ent labels. If this PR is in the CE repo, you should only backport to N, using the backport/x.x.x label, not the enterprise labels.
  • If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
  files.
• Jira: If this change has an associated Jira, it's referenced either
  in the PR description, commit message, or branch name.
• RFC: If this change has an associated RFC, please link it in the description.
• ENT PR: If this change has an associated ENT PR, please link it in the
  description. Also, make sure the changelog is in this PR, not in your ENT PR.

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CI Results:
All Go tests succeeded! ✅

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Build Results:
All builds succeeded! ✅