ext4: remove redundant operation that set bh to NULL

The out_fail branch path don't release the bh and the second bh is
valid only in the for statement, so we don't need to set them to NULL.

Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: zhangyi (F) <yi.zhang@huawei.com>
Link: https://lore.kernel.org/r/1603194069-17557-1-git-send-email-kaixuxia@tencent.com


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