[syzbot] [jfs?] KASAN: slab-use-after-free Read in dbFreeBits

[Edward Adam Davis]

sync jfs remount and  jfs ioctrl with s_umount

#syz test: upstream master

diff --git a/fs/jfs/jfs_dmap.c b/fs/jfs/jfs_dmap.c
index cb3cda1390ad..a409ae18454a 100644
--- a/fs/jfs/jfs_dmap.c
+++ b/fs/jfs/jfs_dmap.c
@@ -1645,7 +1645,9 @@ s64 dbDiscardAG(struct inode *ip, int agno, s64 minlen)
 		 * call jfs_issue_discard() itself */
 		if (!(JFS_SBI(sb)->flag & JFS_DISCARD))
 			jfs_issue_discard(ip, tt->blkno, tt->nblocks);
+		down_read(&sb->s_umount);
 		dbFree(ip, tt->blkno, tt->nblocks);
+		up_read(&sb->s_umount);
 		trimmed += tt->nblocks;
 	}
 	kfree(totrim);