staging/lustre/llite: error of listxattr when buffer is small

According to the standard, listxattr(2) should return -1
and errno should be set to ERANGE if the size of the list
buffer is too small to hold the result. However ll_listxattr()
will return a value bigger than the size of buffer in some cases.

Let's assume listxattr(2) returns SIZE when it is called with a
large enough list buffer. If it's called again with a list buffer
whose size is smaller than SIZE but bigger than (SIZE - 12), then
listxattr(2) will return SIZE too. This patch fixes the problem.
Original patch by Li Xi <pkuelelixi@gmail.com>

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3403
Lustre-change: http://review.whamcloud.com/6463Signed-off-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Li Xi <pkuelelixi@gmail.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/llite/xattr.c

@@ -564,7 +564,12 @@ ssize_t ll_listxattr(struct dentry *dentry, char *buffer, size_t size)
 		const size_t name_len   = sizeof("lov") - 1;
 		const size_t total_len  = prefix_len + name_len + 1;
 
-		if (buffer && (rc + total_len) <= size) {
+		if (((rc + total_len) > size) && (buffer != NULL)) {
+			ptlrpc_req_finished(request);
+			return -ERANGE;
+		}
+
+		if (buffer != NULL) {
 			buffer += rc;
 			memcpy(buffer, XATTR_LUSTRE_PREFIX, prefix_len);
 			memcpy(buffer + prefix_len, "lov", name_len);