iscsi-target: Fix iov_count calculation bug in iscsit_allocate_iovecs

This patch fixes a bug in iscsit_allocate_iovecs() where iov_count was
incorrectly calculated using min(1UL, data_length / PAGE_SIZE) instead of
max(1UL, data_length / PAGE_SIZE), that ends up triggering an OOPs for
large block I/O when the SGL <-> iovec mapping exceeds the bogus iov_count
allocation size.

This is a regression introduced during the iscsi-target conversion back
to using core memory allocation here:

commit bfb79eac20
Author: Andy Grover <agrover@redhat.com>
Date:   Tue Apr 3 15:51:29 2012 -0700

    target/iscsi: Go back to core allocating data buffer for cmd

Cc: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target.c

@@ -760,7 +760,7 @@ static void iscsit_ack_from_expstatsn(struct iscsi_conn *conn, u32 exp_statsn)
 
 static int iscsit_allocate_iovecs(struct iscsi_cmd *cmd)
 {
-	u32 iov_count = min(1UL, DIV_ROUND_UP(cmd->se_cmd.data_length, PAGE_SIZE));
+	u32 iov_count = max(1UL, DIV_ROUND_UP(cmd->se_cmd.data_length, PAGE_SIZE));
 
 	iov_count += ISCSI_IOV_DATA_BUFFER;