ext4: fix incorrect locking in move_extent_per_page

If we have to copy data we must drop i_data_sem because of
get_blocks() will be called inside mext_page_mkuptodate(), but later we must
reacquire it again because we are about to change extent's tree
Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

fs/ext4/move_extent.c

@@ -1013,10 +1013,11 @@ move_extent_per_page(struct file *o_filp, struct inode *donor_inode,
 		*err = -EBUSY;
 		goto unlock_pages;
 	}
-
+	ext4_double_down_write_data_sem(orig_inode, donor_inode);
 	replaced_count = mext_replace_branches(handle, orig_inode, donor_inode,
 					       orig_blk_offset,
 					       block_len_in_page, err);
+	ext4_double_up_write_data_sem(orig_inode, donor_inode);
 	if (*err) {
 		if (replaced_count) {
 			block_len_in_page = replaced_count;