Restore original array_rd_sz semantics.

Before r278702 prefetch was blocked for I/Os > 1MB, after -- >= 1MB.
1MB I/Os are used for bulk operations in CTL (XCOPY, VERIFY), and disabling
prefetch for them reduced the performance.

This is temporary local patch, that should be replaced when upstreamed.

Discussed with: mahrens
MFC after: 3 days