vfs: fs_context: fix up param length parsing in legacy_parse_param

The "PAGE_SIZE - 2 - size" calculation in legacy_parse_param() is an
unsigned type so a large value of "size" results in a high positive
value instead of a negative value as expected. Fix this by getting rid
of the subtraction.

Signed-off-by: Jamie Hill-Daniel <[e-mail]>
Signed-off-by: William Liu <[e-mail]>
Tested-by: Salvatore Bonaccorso <[e-mail]>
Tested-by: Thadeu Lima de Souza Cascardo <[e-mail]>
Acked-by: Dan Carpenter <[e-mail]>
Acked-by: Al Viro <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/fs_context.c

···
 			      param->key);
 	}
 
-	if (len > PAGE_SIZE - 2 - size)
+	if (size + len + 2 > PAGE_SIZE)
 		return invalf(fc, "VFS: Legacy: Cumulative options too large");
 	if (strchr(param->key, ',') ||
 	    (param->type == fs_value_is_string &&