sync jfs remount and  jfs ioctrl with s_umount

#syz test: upstream master

diff --git a/fs/jfs/jfs_discard.c b/fs/jfs/jfs_discard.c
index 575cb2ba74fc..5f4b305030ad 100644
--- a/fs/jfs/jfs_discard.c
+++ b/fs/jfs/jfs_discard.c
@@ -65,7 +65,7 @@ void jfs_issue_discard(struct inode *ip, u64 blkno, u64 nblocks)
 int jfs_ioc_trim(struct inode *ip, struct fstrim_range *range)
 {
 	struct inode *ipbmap = JFS_SBI(ip->i_sb)->ipbmap;
-	struct bmap *bmp = JFS_SBI(ip->i_sb)->bmap;
+	struct bmap *bmp;
 	struct super_block *sb = ipbmap->i_sb;
 	int agno, agno_end;
 	u64 start, end, minlen;
@@ -83,10 +83,15 @@ int jfs_ioc_trim(struct inode *ip, struct fstrim_range *range)
 	if (minlen == 0)
 		minlen = 1;
 
+	down_read(&sb->s_umount);
+	bmp = JFS_SBI(ip->i_sb)->bmap;
+
 	if (minlen > bmp->db_agsize ||
 	    start >= bmp->db_mapsize ||
-	    range->len < sb->s_blocksize)
+	    range->len < sb->s_blocksize) {
+		up_read(&sb->s_umount);
 		return -EINVAL;
+	}
 
 	if (end >= bmp->db_mapsize)
 		end = bmp->db_mapsize - 1;
@@ -100,6 +105,8 @@ int jfs_ioc_trim(struct inode *ip, struct fstrim_range *range)
 		trimmed += dbDiscardAG(ip, agno, minlen);
 		agno++;
 	}
+
+	up_read(&sb->s_umount);
 	range->len = trimmed << sb->s_blocksize_bits;
 
 	return 0;

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+3c010e21296f33a5dc16@syzkaller.appspotmail.com
Tested-by: syzbot+3c010e21296f33a5dc16@syzkaller.appspotmail.com

Tested on:

commit:         60f0560f Merge tag 'nfs-for-6.11-2' of git://git.linux..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1159e82b980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4fc2afd52fd008bb
dashboard link: https://syzkaller.appspot.com/bug?extid=3c010e21296f33a5dc16
compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
patch:          https://syzkaller.appspot.com/x/patch.diff?x=12dc9bd5980000

Note: testing is done by a robot and is best-effort only.