fs/tar: only store numeric uid/gid

If a target user is asigned a UID (e.g. 1000) that happens to also exist
on the build machine, tar will happily store the username for that user.

This can be seen by some as potential information disclosure.

Instruct tar to just store the numeric uid/gid.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>