fsnotify: Fix NULL ptr deref in fanotify_get_fsid()

fanotify_get_fsid() is reading mark->connector->fsid under srcu. It can
happen that it sees mark not fully initialized or mark that is already
detached from the object list. In these cases mark->connector
can be NULL leading to NULL ptr dereference. Fix the problem by
being careful when reading mark->connector and check it for being NULL.
Also use WRITE_ONCE when writing the mark just to prevent compiler from
doing something stupid.

Reported-by:  <syzbot+15927486a4f1bfcbaf91@syzkaller.appspotmail.com>
Fixes: 77115225

 ("fanotify: cache fsid in fsnotify_mark_connector")
Signed-off-by: Jan Kara <jack@suse.cz>