i387: use 'restore_fpu_checking()' directly in task switching code

This inlines what is usually just a couple of instructions, but more
importantly it also fixes the theoretical error case (can that FPU
restore really ever fail? Maybe we should remove the checking).

We can't start sending signals from within the scheduler, we're much too
deep in the kernel and are holding the runqueue lock etc.  So don't
bother even trying.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>