KVM: x86: Avoid double RDPKRU when loading host/guest PKRU

KVM: x86: Avoid double RDPKRU when loading host/guest PKRU

Use the raw wrpkru() helper when loading the guest/host's PKRU on switch
to/from guest context, as the write_pkru() wrapper incurs an unnecessary
rdpkru().  In both paths, KVM is guaranteed to have performed RDPKRU since
the last possible write, i.e. KVM has a fresh cache of the current value
in hardware.

This effectively restores KVM's behavior to that of KVM prior to commit
c806e88734b9 ("x86/pkeys: Provide *pkru() helpers"), which renamed the raw
helper from __write_pkru() => wrpkru(), and turned __write_pkru() into a
wrapper.  Commit 577ff465f5a6 ("x86/fpu: Only write PKRU if it is different
from current") then added the extra RDPKRU to avoid an unnecessary WRPKRU,
but completely missed that KVM already optimized away pointless writes.

Reported-by: Adrian Hunter <adrian.hunter@intel.com>
Fixes: 577ff465f5a6 ("x86/fpu: Only write PKRU if it is different from current")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20241221011647.3747448-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>