Simplify synchronization read error handling.

Since synchronization reads are performed by submitting a request to
the external mirror provider, we know that the request returns with an
error only when gmirror was unable to read a copy of the block from any
mirror. Thus, there is no need to retry the request from the
synchronization error handler.

Tested by: pho
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon