isofs: delete unnecessary checks before brelse()

The brelse() function tests whether its argument is NULL
and then returns immediately.
Thus remove the tests which are not needed around the shown calls.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Minghao Chi <chi.minghao@zte.com.cn>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220819081420.96209-1-chi.minghao@zte.com.cn