x86: mm/fault: Fix semaphore imbalance

x86: mm/fault: Fix semaphore imbalance

When backporting commit 33692f27597f ('vm: add VM_FAULT_SIGSEGV
handling support') I didn't notice that it depended on a recent change
to the locking context of mm_fault_error() (commit 7fb08eca4527,
'x86: mm: move mmap_sem unlock from mm_fault_error() to caller').
That isn't easily applicable to 3.2, so instead make sure we drop
mm->mmap_sem on the new branch of mm_fault_error().

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>