fsnotify: fix false positive warning on inode delete

When inode is getting deleted and someone else holds reference to a mark
attached to the inode, we just detach the connector from the inode. In
that case fsnotify_put_mark() called from fsnotify_destroy_marks() will
decide to recalculate mask for the inode and __fsnotify_recalc_mask()
will WARN about invalid connector type:

WARNING: CPU: 1 PID: 12015 at fs/notify/mark.c:139
__fsnotify_recalc_mask+0x2d7/0x350 fs/notify/mark.c:139

Actually there's no reason to warn about detached connector in
__fsnotify_recalc_mask() so just silently skip updating the mask in such
case.

Reported-by:  <syzbot+c34692a51b9a6ca93540@syzkaller.appspotmail.com>
Fixes: 3ac70bfc ("fsnotify: add helper to get mask from connector")
Signed-off-by: Jan Kara <jack@suse.cz>