swap_pager: fix seek_data with invalid first page

[dougm]

Summary

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.

Test Plan

[e-mail] reported the problem, and reports that this change resolved it.

[markj]

This was broken by db08b0b04deced766c3b5f07bcfb82333666226c.

In mail thread, @des suggests that the problem exists on stable/14 as well, but the above-mentioned commit is not present there, so at least one of the two claims is wrong. Which one is it?

[des]

In D47767#1089263, @markj wrote:

In mail thread, @des suggests that the problem exists on stable/14 as well,

No, I said it occurs in a 14 jail on a 15 host. It does not occur in a 13 jail because 13's install(1) does not use copy_file_range(2) and therefore does not trip over the bug in the host's 15 kernel.