ffs: fix vn_io_fault_pgmove() offset for PAGE_SIZE > block size

The "offset" argument to vn_io_fault_pgmove() is supposed to be
the offset within the page, but for ffs we currently use the offset
within the block. When the block size is at least as large as the
page size then these values are the same, but when the page size is
larger than the block size then we need to add the offset of
the block within the page as well.

Sponsored by: Netflix

Reviewed by: mckusick, kib, markj
Differential Revision: https://reviews.freebsd.org/D34835