swap_pager: Reduce the scope of the object lock in putpages

We don't need to hold the object lock while allocating swap space, so
don't.

Reviewed by: dougm, kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D35839

(cherry picked from commit 54291f7d6506e6c6087433c5bbdb2224b6cef23b)