swap_pager_freespace: fix freed count

Function swp_pager_meta_transfer uses 'pindex' as the start address of
the swblk in calculating which page to lookup in order to count freed
pages. However, the lookup for a swblk at 'pindex' or greater may
produce one greater than 'pindex', given by sb->p, and that's the
value that should be used to compute a page adddress.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D46234