Restore an optimization that was temporary disabled by r324665.

In reclaim_pv_chunk(), rotate the pv chunks list so that next
invocations of the reclaim do not scan the same pv chunks that could
not be freed. Only do the rotation when there is no parallel scan,
tracked by active_reclaims counter.

To rotate, move all chunks that are before current iteration marker,
after another marker that is inserted at the list tail on start of the
reclaim.

Reviewed by: alc
Tested by: pho (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week