zfs: backport linux 5.8 compatibility patch to 0.8.4 release

[rtreffer]

Adding this patch makes zfs compile against 5.8-rc7 kernels.

Upstream PR (merged): openzfs/zfs#10422
Modifications:

• spl_kvmalloc does not exists in zfs 0.8.4 - related code was dropped.

Motivation for this change

I am trying to build a 5.8 kernel and zfs did not build.
There is an upstream PR that added a thin define around __vmalloc to support both the old and new interface.

This will probably turn into an issue once 5.8 is released and promoted from linuxPackages_testing.

Things done

I have build a small nixos iso image with this nixpkgs version and successfully created a zfs backed mount with the updated kernel package (kernel 5.4.54).

I have then switched the kernel to the linuxPackages_testing and created a new zfs volume (kernel 5.8-rc7).

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[Mic92]

@GrahamcOfBorg test zfs

[Ma27]

@rtreffer thanks!