Coalesce socket reads in software iSCSI.

Instead of 2-4 socket reads per PDU this can do as low as one read
per megabyte, dramatically reducing TCP overhead and lock contention.

With this on iSCSI target I can write more than 4GB/s through a
single connection.

MFC after: 1 month

(cherry picked from commit 6895f89fe54e0858aea70d2bd2a9651f45d7998e)