Introduce "soft" serseq variant.

With new ZFS prefetcher improvements it is no longer needed to fully
serialize reads to reach decent prediction hit rate. Softer variant
only creates small time window to reduce races instead of completely
blocking following reads while previous is running. It much less
hurts the performance in case of prediction miss.

MFC after: 1 month

(cherry picked from commit ac503c194cd8e9dbef5c120e87f9521e1a89003a)