The pa argument for sendfile_iodone() is not necessary a slice of sfio->pa.

It is true for zfs, but it is not for e.g. vnode or buffer pagers.
When fixing bogus pages, fix them in both places. Rely on the fact
that pa[0] must have been invalid so it cannot be bogus.

Reported and tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks