mbuf: Fix an offset calculation in m_apply_extpg_one()

We were not including the requested starting offset in the page offset.

Approved by: re (cperciva)
Reviewed by: jhb
Fixes: 3c7a01d773ac ("Extend m_apply() to support unmapped mbufs.")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32922

(cherry picked from commit ac2b54441742b3148de72e3b29aa0f00d29bb825)
(cherry picked from commit 70421fd527e315bbc4850db2d7e80fc008393bce)