Move the code to destroy half-contructed nullfs vnode into helper
function null_destroy_proto() from null_insmntque_dtr(). Also
apply null_destroy_proto() in null_nodeget() when we raced and a vnode
is found in the hash, so the currently allocated protonode shall be
destroyed.

Lock the vnode interlock around reassigning the v_vnlock.

In fact, this path will not be exercised after several later commits,
since null_nodeget() cannot take shared-locked lowervp at all due to
insmntque() requirements.

Reported by: rea
Tested by: pho
MFC after: 1 week