nullfs: provide custom bypass for VOP_READ_PGCACHE().

Normal bypass expects locked vnode, which is not true for
VOP_READ_PGCACHE(). Ensure liveness of the lower vnode by taking the
upper vnode interlock, which is also taked by null_reclaim() when
setting v_data to NULL.

Reported and tested by: pho
Reviewed by: markj, mjg
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D27327