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>

Author: LiBaokun96

fs/ext4/ext4.h

@@ -1631,7 +1631,6 @@ struct ext4_sb_info {
 	unsigned int s_max_dir_size_kb;
 	/* where last allocation was done - for stream allocation */
 	unsigned long s_mb_last_group;
-	unsigned long s_mb_last_start;
 	unsigned int s_mb_prefetch;
 	unsigned int s_mb_prefetch_limit;
 	unsigned int s_mb_best_avail_max_trim_order;

fs/ext4/mballoc.c

@@ -2171,7 +2171,6 @@ static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
 	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
 		spin_lock(&sbi->s_md_lock);
 		sbi->s_mb_last_group = ac->ac_f_ex.fe_group;
-		sbi->s_mb_last_start = ac->ac_f_ex.fe_start;
 		spin_unlock(&sbi->s_md_lock);
 	}
 	/*