ext4: remove unnecessary s_mb_last_start

Since stream allocation does not use ac->ac_f_ex.fe_start, it is set to -1
by default, so the no longer needed sbi->s_mb_last_start is removed.

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20250714130327.1830534-4-libaokun1@huawei.com


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