libsmb: fix regression on smbc_getxattr and fix doc

Bug: https://bugzilla.samba.org/show_bug.cgi?id=14808
Signed-off-by: Remi Collet <rcollet@redhat.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>

source3/include/libsmbclient.h

@@ -2419,7 +2419,7 @@ int smbc_getxattr(const char *url,
  *                  required to hold the attribute value will be returned,
  *                  but nothing will be placed into the value buffer.
  *
- * @return          0 on success, < 0 on error with errno set:
+ * @return          size on success, < 0 on error with errno set:
  *                  - EINVAL  The client library is not properly initialized
  *                            or one of the parameters is not of a correct
  *                            form

source3/libsmb/libsmb_xattr.c

@@ -2182,9 +2182,9 @@ SMBC_getxattr_ctx(SMBCCTX *context,
 		TALLOC_FREE(frame);
 		/*
 		 * static function cacl_get returns a value greater than zero
-		 * on success. Map this to zero meaning success.
+		 * which is needed buffer size needed when size_t is 0.
 		 */
-                return ret < 0 ? -1 : 0;
+                return ret;
         }
 
         /* Unsupported attribute name */