Btrfs: fix comparison in __btrfs_map_block()

Add missing comparison to op in expression, which was forgotten when doing
the REQ_OP transition.

Fixes: b3d3fa51 ("btrfs: update __btrfs_map_block for REQ_OP transition")
Signed-off-by: Vincent Stehlé <vincent.stehle@intel.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

fs/btrfs/volumes.c

@@ -5494,7 +5494,7 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info, int op,
 		}
 
 	} else if (map->type & BTRFS_BLOCK_GROUP_DUP) {
-		if (op == REQ_OP_WRITE || REQ_OP_DISCARD ||
+		if (op == REQ_OP_WRITE || op == REQ_OP_DISCARD ||
 		    op == REQ_GET_READ_MIRRORS) {
 			num_stripes = map->num_stripes;
 		} else if (mirror_num) {