ext4: add missed brelse in ext4_free_blocks_simple

Release bitmap buffer_head we got if error occurs.
Besides, this patch remove unused assignment to err.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://lore.kernel.org/r/20230303172120.3800725-8-shikemeng@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>