fs/ntfs3: Fix NULL deref in ntfs_update_mftmirr

fs/ntfs3: Fix NULL deref in ntfs_update_mftmirr

If ntfs_fill_super() wasn't called then sbi->sb will be equal to NULL.
Code should check this ptr before dereferencing. Syzbot hit this issue
via passing wrong mount param as can be seen from log below

Fail log:
ntfs3: Unknown parameter 'iochvrset'
general protection fault, probably for non-canonical address 0xdffffc0000000003: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
CPU: 1 PID: 3589 Comm: syz-executor210 Not tainted 5.18.0-rc3-syzkaller-00016-gb253435746d9 #0
...
Call Trace:
 <TASK>
 put_ntfs+0x1ed/0x2a0 fs/ntfs3/super.c:463
 ntfs_fs_free+0x6a/0xe0 fs/ntfs3/super.c:1363
 put_fs_context+0x119/0x7a0 fs/fs_context.c:469
 do_new_mount+0x2b4/0xad0 fs/namespace.c:3044
 do_mount fs/namespace.c:3383 [inline]
 __do_sys_mount fs/namespace.c:3591 [inline]

Fixes: 82cae269cfa9 ("fs/ntfs3: Add initialization of super block")
Reported-and-tested-by: syzbot+c95173762127ad76a824@syzkaller.appspotmail.com
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>