x86/fpu: Rename fregs-related copy functions

The function names for fnsave/fnrstor operations are horribly named and
a permanent source of confusion.

Rename:
	copy_kernel_to_fregs() to frstor()
	copy_fregs_to_user()   to fnsave_to_user_sigframe()
	copy_user_to_fregs()   to frstor_from_user_sigframe()

so it's clear what these are doing. All these functions are really low
level wrappers around the equally named instructions, so mapping to the
documentation is just natural.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121454.223594101@linutronix.de