Allow swap_pager_putpages() to allocate one block at a time.

[markj]

Summary

Alan points out that the minimum allocation size of 4 blocks is an old
policy that came with the "new" swap pager in r42957. Since then the
blist allocator has gotten better at minimizing fragmentation; for
example, with r349777 it can return a range that spans multiple leaves.
When swap space is close to being exhaused, the minimum of 4 blocks most
likely exacerbates things. Reduce it to 1.

Test Plan

Peter reported no problems.

[dougm]

This change makes the second argument to swp_pager_getswapspace redundant. A future change might as well eliminate it.

[markj]

In D23763#522914, @dougm wrote:

This change makes the second argument to swp_pager_getswapspace redundant. A future change might as well eliminate it.

Indeed. I didn't bother since that change doesn't really simplify swp_pager_getswapspace() at all, but it seems reasonable to me.