keys: Add tracepoints for the keyrings facility

Add some tracepoints to aid in debuggin the keyrings facility and
applications that use it.  A number of events and operations are traceable,
including:

  - Allocation
  - Refcounting
  - Instantiation and negative instantiation/rejection
  - Update
  - Detection of key being dead
  - Key quota changes
  - Key quota failure
  - Link, unlink and move
  - Keyring clearance
  - Revocation and invalidation
  - Garbage collection

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jarkko Sakkinen <jarkko@kernel.org>
cc: keyrings@vger.kernel.org
cc: linux-security-module@vger.kernel.org