powerpc/mm: convert coprocessor fault to lock_mm_and_find_vma()

This is one of the simple cases, except there's no pt_regs pointer.
Which is fine, as lock_mm_and_find_vma() is set up to work fine with a
NULL pt_regs.

Powerpc already enabled LOCK_MM_AND_FIND_VMA for the main CPU faulting,
so we can just use the helper without any extra work.

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