Correct uma_prealloc()'s use of domainset iterators after r339925.

The iterator should be reinitialized after every successful slab
allocation. A request to advance the iterator is interpreted as
an allocation failure, so a sufficiently large preallocation would
cause the iterator to believe that all domains were exhausted,
resulting in a sleep with the keg lock held. [1]

Also, keg_alloc_slab() should pass the unmodified wait flag to the
item initialization routine, which may use it to perform allocations
from other zones.

Reported and tested by: slavah
Diagnosed by: kib [1]
Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation