jbd2: remove dead check of JBD2_UNMOUNT in kjournald2

jbd2: remove dead check of JBD2_UNMOUNT in kjournald2

We always set JBD2_UNMOUNT with j_state_lock held in journal_kill_thread.
In kjournald2, we check JBD2_UNMOUNT flag two times under the same
j_state_lock. Then the second check is unnecessary.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20240514112438.1269037-9-shikemeng@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>