vfs: introduce helper function to safely NUL-terminate symlinks

A number of filesystems were potentially triggering kernel bugs due to
corrupted symlink names on disk. This function helps safely terminate
the names.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: NDuane Griffin <duaneg@dghda.com>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

include/linux/namei.h

@@ -94,4 +94,9 @@ static inline char *nd_get_link(struct nameidata *nd)
 	return nd->saved_names[nd->depth];
 }
 
+static inline void nd_terminate_link(void *name, size_t len, size_t maxlen)
+{
+	((char *) name)[min(len, maxlen)] = '\0';
+}
+
 #endif /* _LINUX_NAMEI_H */