target: Fix >= v3.9+ regression in PR APTPL + ALUA metadata write-out

This patch fixes a >= v3.9+ regression in __core_scsi3_write_aptpl_to_file()
+ core_alua_write_tpg_metadata() write-out, where a return value of -EIO was
incorrectly being returned upon success.

This bug was originally introduced in:

commit 0e9b10a90f1c30f25dd6f130130240745ab14010
Author: Al Viro <viro@zeniv.linux.org.uk>
Date:   Sat Feb 23 15:22:43 2013 -0500

    target: writev() on single-element vector is pointless

However, given that the return of core_scsi3_update_and_write_aptpl()
was not used to determine if a command should be returned with non GOOD
status, this bug was not being triggered in PR logic until v3.11-rc1 by
commit:

commit 459f213ba162bd13e113d6f92a8fa6c780fd67ed
Author: Andy Grover <agrover@redhat.com>
Date:   Thu May 16 10:41:02 2013 -0700

    target: Allocate aptpl_buf inside update_and_write_aptpl()

So, go ahead and only return -EIO if kernel_write() returned a
negative value.

Reported-by: Gera Kazakov <gkazakov@msn.com>
Signed-off-by: Gera Kazakov <gkazakov@msn.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andy Grover <agrover@redhat.com>
Cc: stable@vger.kernel.org # 3.9+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_alua.c

@@ -733,7 +733,7 @@ static int core_alua_write_tpg_metadata(
 	if (ret < 0)
 		pr_err("Error writing ALUA metadata file: %s\n", path);
 	fput(file);
-	return ret ? -EIO : 0;
+	return (ret < 0) ? -EIO : 0;
 }
 
 /*

drivers/target/target_core_pr.c

@@ -1949,7 +1949,7 @@ static int __core_scsi3_write_aptpl_to_file(
 		pr_debug("Error writing APTPL metadata file: %s\n", path);
 	fput(file);
 
-	return ret ? -EIO : 0;
+	return (ret < 0) ? -EIO : 0;
 }
 
 /*