swap_pager: fix seek_data with invalid first page

Correct swap_pager_seek_data so that, when the first lookup finds no
valid pages, second and subsequent lookups are attempted anyway.

This was broken by db08b0b04deced766c3b5f07bcfb82333666226c.

Reported by: marklmi@yahoo.com
Reviewed by: kib
Tested by: marklmi@yahoo.com
Fixes: db08b0b04deced76 tmpfs_vnops: move swap work to swap_pager
Differential Revision: https://reviews.freebsd.org/D47767