ext4: avoid possible double brelse() in add_new_gdb() on error path

commit 4f32c38b upstream.

Fixes: b4097142 ("ext4: add error checking to calls to ...")
Reported-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org # 2.6.38
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>