tmpfs_rename(): use tmpfs_access_locked instead of VOP_ACCESS()

Protect the call with the node lock. We cannot lock the fvp vnode
sleepable there, because we already own other participating vnode's
locks. Taking it without sleeping require unwinding the whole locking
state in one more place.

Note that the liveness of the node is guaranteed by the lock on the
parent directory vnode.

Reported and tested by: pho
Fixes: cbac1f3464956185cf95955344b6009e2cc3ae40ESC
Reviewed by: markj, mjg
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D38557