x86/fpu/signal: Move initial checks into fpu__restore_sig()

__fpu__restore_sig() is convoluted and some of the basic checks can
trivially be done in the calling function as well as the final error
handling of clearing user state.

 [ bp: Fixup typos. ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121457.086336154@linutronix.de