bcachefs: Fix dio write path with loopback dio mode

When the iov_iter is a bvec iter, it's possible the IO was submitted
from a kthread that didn't have an mm to switch to.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/fs-io.c

@@ -2110,7 +2110,7 @@ static long bch2_dio_write_loop(struct dio_write *dio)
 	while (1) {
 		iter_count = dio->iter.count;
 
-		if (kthread)
+		if (kthread && dio->mm)
 			kthread_use_mm(dio->mm);
 		BUG_ON(current->faults_disabled_mapping);
 		current->faults_disabled_mapping = mapping;
@@ -2120,7 +2120,7 @@ static long bch2_dio_write_loop(struct dio_write *dio)
 		dropped_locks = fdm_dropped_locks();
 
 		current->faults_disabled_mapping = NULL;
-		if (kthread)
+		if (kthread && dio->mm)
 			kthread_unuse_mm(dio->mm);
 
 		/*