ext4: add missing brelse() in set_flexbg_block_bitmap()'s error path

Fixes: 33afdcc5

 ("ext4: add a function which sets up group blocks ...")
Cc: stable@kernel.org # 3.3
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>