Linux 4.4 compat: make_request_fn returns blk_qc_t

As part of block polling support in Linux 4.4, make_request_fn should
return a cookie value of type blk_qc_t. For now, we make zvol_request
always return BLK_QC_T_NONE until we assess whether and how we want
to support block polling.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #4021

module/zfs/zvol.c

@@ -755,6 +755,8 @@ out1:
 	spl_fstrans_unmark(cookie);
 #ifdef HAVE_MAKE_REQUEST_FN_RET_INT
 	return (0);
+#elif defined(HAVE_MAKE_REQUEST_FN_RET_QC)
+	return (BLK_QC_T_NONE);
 #endif
 }