ext4: use correct depth value

Inode's depth can be changed from here:
ext4_ext_try_to_merge() ->ext4_ext_try_to_merge_up()
We must use correct value.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>