jbd2: remove repeated assignments in __jbd2_log_wait_for_space()

mainline inclusion
from mainline-5.2-rc1
commit fb203751
category: bugfix
bugzilla: 25031
CVE: NA
---------------------------

At the beginning, nblocks has been assigned. There is no need
to repeat the assignment in the while loop, and remove it.

Signed-off-by: Liu Song <liu.song11@zte.com.cn>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Reviewed-by: Yang Erkun <yangerkun@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>