Fix a swap block allocation race.

Fix a swap block allocation race.

putpages' allocation of swap blocks is done under the global sw_dev
lock.  Previously it would drop that lock before inserting the allocated
blocks into the object's trie, creating a window in which swap blocks
are allocated but are not visible to swapoff.  This can cause
swp_pager_strategy() to fail and panic the system.

Fix the problem bluntly, by allocating swap blocks under the object
lock.

Reviewed by:	jeff, kib
Tested by:	pho
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D23665