f2fs: fix error path handling in truncate_dnode()

If truncate_node() fails in truncate_dnode(), it missed to call
f2fs_put_page(), fix it.

Fixes: 7735730d ("f2fs: fix to propagate error from __get_meta_page()")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>