sparc32: fix lock_mm_and_find_vma() conversion

The sparc32 conversion to lock_mm_and_find_vma() in commit a050ba1e


("mm/fault: convert remaining simple cases to lock_mm_and_find_vma()")
missed the fact that we didn't actually have a 'regs' pointer available
in the 'force_user_fault()' case.

It's there in the regular page fault path ("do_sparc_fault()"), but not
the window underflow/overflow paths.

Which is all fine - we can just pass in a NULL pointer.  The register
state is only used to avoid deadlock with kernel faults, which is not
the case for any of these register window faults.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: a050ba1e

 ("mm/fault: convert remaining simple cases to lock_mm_and_find_vma()")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>