Add -P option to preserve file owner

Add -P option to preserve file owner

I use user namespaces & debootstrap to bootstrap a system, and then
copy that into an image using sload.f2fs. However, without an option to
preserve the owner user & group of files, some files end up being owned
by the wrong user / group (0:0), which causes all kinds of other issues

This patch adds an option -P to preserve the user and group of files.

Signed-off-by: Daniel Abrecht <public@danielabrecht.ch>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>