bcachefs; Use trans_unlock_long() when waiting on allocator

not using unlock_long() blocks key cache reclaim, and the allocator may
take awhile
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/io_misc.c

@@ -125,7 +125,7 @@ int bch2_extent_fallocate(struct btree_trans *trans,
 	bch2_bkey_buf_exit(&old, c);
 
 	if (closure_nr_remaining(&cl) != 1) {
-		bch2_trans_unlock(trans);
+		bch2_trans_unlock_long(trans);
 		closure_sync(&cl);
 	}