ext4: add i_data_sem sanity check

Each caller of ext4_ext_dirty must hold i_data_sem,
The only exception is migration code, let's make it convenient.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index b30172d..ee93f82 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -161,6 +161,8 @@
 		     struct inode *inode, struct ext4_ext_path *path)
 {
 	int err;
+
+	WARN_ON(!rwsem_is_locked(&EXT4_I(inode)->i_data_sem));
 	if (path->p_bh) {
 		ext4_extent_block_csum_set(inode, ext_block_hdr(path->p_bh));
 		/* path points to block */