vn_lock_pair(): allow lkflags1/lkflags2 to be 0 if vp1/vp2 is NULL

It's a bit strange to require the caller to pass contrived lock flags
if the corresponding vnode is NULL, simply to appease the assertion
that exactly one of LK_SHARED or LK_EXCLUSIVE must be set. On the
other hand, we still want to catch cases in which completely bogus
or corrupt flags are passed even if the corresponding vnode is NULL.
Therefore, specifically allow empty flags for lkflags1/lkflags2 iff
the respective vp1/vp2 param is NULL.

Reviewed by: kib, olce
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D44046