p9fs: Call VN_LOCK_ASHARE on new vnodes

We set MNTK_LOOKUP_SHARED on p9fs mounts, but disable shared locking of
vnodes (i.e., LK_SHARED requests are automatically translated to
LK_EXCLUSIVE.

Reviewed by: kib
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D50759