ext4: add missing brelse() add_new_gdb_meta_bg()'s error path

Fixes: 01f795f9

 ("ext4: add online resizing support for meta_bg ...")
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org # 3.7