Assert that the refcount value is not VPRC_BLOCKED in vm_page_drop().

VPRC_BLOCKED is a refcount flag used to indicate that a thread is
tearing down mappings of a page. When set, it causes attempts to wire a
page via a pmap lookup to fail. It should never represent the last
reference to a page, so assert this.

Suggested by: kib
Reviewed by: alc, kib
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D21639