When swap pager allocates metadata in the pagedaemon context, allow it
to drain the reserve.  This was broken in r243040, causing deadlock.
Note that VM_WAIT call in case of uma_zalloc() failure from pagedaemon
would only wait for the v_pageout_free_min anyway.

Reported and tested by:	pho
Reviewed by:	alc
Sponsored by:	The FreeBSD Foundation