IB/umem: Fix possible hang on process exit

If ib_umem_release() is called after ib_uverbs_close() sets context->closing,
then a process can get stuck in a D state, because the code boils down to

	if (down_write_trylock(&mm->mmap_sem))
		down_write(&mm->mmap_sem);

which is obviously a stupid instant deadlock.  Fix the code so that we
only try to take the lock once.

This bug was introduced in commit f7c6a7b ("IB/uverbs: Export
ib_umem_get()/ib_umem_release() to modules") which fortunately never
made it into a release, and was reported by Pete Wyckoff <pw@osc.edu>.

Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/core/umem.c

@@ -225,13 +225,15 @@ void ib_umem_release(struct ib_umem *umem)
 	 * up here and not be able to take the mmap_sem.  In that case
 	 * we defer the vm_locked accounting to the system workqueue.
 	 */
-	if (context->closing && !down_write_trylock(&mm->mmap_sem)) {
-		INIT_WORK(&umem->work, ib_umem_account);
-		umem->mm   = mm;
-		umem->diff = diff;
-
-		schedule_work(&umem->work);
-		return;
+	if (context->closing) {
+		if (!down_write_trylock(&mm->mmap_sem)) {
+			INIT_WORK(&umem->work, ib_umem_account);
+			umem->mm   = mm;
+			umem->diff = diff;
+
+			schedule_work(&umem->work);
+			return;
+		}
 	} else
 		down_write(&mm->mmap_sem);