Extract tmpfs-specific part of tmpfs_access() into a helper

The helper tmpfs_access_locked() requires either the vnode or node
locked for consistency of the access check, unlike the pure vnode op.

Reviewed by: markj, mjg
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D38557