vfs_domount(): ensure that v_mountedhere and VIRF_MOUNTPOINT are set under the vnode lock

Fixes: f7833196bd6ba9bfc060a41b353422b15d6aa95b
Reported and tested by: pho
Reviewed by: jah, markj (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D37198