x86: mm/fault: Fix semaphore imbalance

When backporting commit 33692f27 ('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 7fb08eca,
'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>

arch/x86/mm/fault.c

@@ -878,7 +878,7 @@ mm_fault_error(struct pt_regs *regs, unsigned long error_code,
 			     VM_FAULT_HWPOISON_LARGE))
 			do_sigbus(regs, error_code, address, fault);
 		else if (fault & VM_FAULT_SIGSEGV)
-			bad_area_nosemaphore(regs, error_code, address);
+			bad_area(regs, error_code, address);
 		else
 			BUG();
 	}