Only update the domain cursor once in keg_fetch_slab().

Only update the domain cursor once in keg_fetch_slab().

We drop the keg lock when we go to actually allocate the slab, allowing
other threads to advance the cursor.  This can cause us to exit the
round-robin loop before having attempted allocations from all domains,
resulting in a hang during a subsequent blocking allocation attempt from
a depleted domain.

Reported and tested by:	Jan Bramkamp <crest@bultmann.eu>
Reviewed by:	alc, cem
Approved by:	re (gjb)
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D17209