x86/fpu: Update fpu_swap_kvm_fpu() uses in comments as well

The following commit:

  d69c1382 ("x86/kvm: Convert FPU handling to a single swap buffer")

reworked KVM FPU handling, but forgot to update the comments
in xstate_op_valid(): fpu_swap_kvm_fpu() doesn't exist anymore,
fpu_swap_kvm_fpstate() is used instead.

Update the comments accordingly.

[ mingo: Improved the changelog. ]

Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240403091803.818-1-lirongqing@baidu.com