uma: Avoid polling for an invalid SMR sequence number

Buckets in an SMR-enabled zone can legitimately be tagged with
SMR_SEQ_INVALID. This effectively means that the zone destructor (if
any) was invoked on all items in the bucket, and the contained memory is
safe to reuse. If the first bucket in the full bucket list was tagged
this way, UMA would unnecessarily poll per-CPU state before attempting
to fetch a full bucket from the list.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit a04ce833f9ba058ec3e04f686bd56fa0691bfcf0)