inotify: Fix a couple of locking bugs

When hooking vop_rename_post(), the preexisting dst vnode will be
unlocked. But, we need to invoke VOP_GETATTR on it in vn_inotify() to
check its link count.

In inotify_unlink_watch_locked(), the vnode interlock is not held, so
don't use vn_irflag_unset_locked().

Reviewed by: kib
Fixes: f1f230439fa4 ("vfs: Initial revision of inotify")
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D51401