x86/fpu: Update the outdated comment above fpstate_init_user()

fpu_init_fpstate_user() was removed in:

  commit 582b01b6 ("x86/fpu: Remove old KVM FPU interface").

Update that comment to accurately reflect the current state regarding its
callers.

Signed-off-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250324131931.2097905-1-chao.gao@intel.com