ufs_get_locked_page(): make sure we have buffer_heads

callers rely upon that, but find_lock_page() racing with attempt of
page eviction by memory pressure might have left us with
	* try_to_free_buffers() successfully done
	* __remove_mapping() failed, leaving the page in our mapping
	* find_lock_page() returning an uptodate page with no
buffer_heads attached.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>