xfs_protofile: fix permission octet when suid/guid is set

xfs_protofile: fix permission octet when suid/guid is set

When encountering suid or sgid files, we already set the `u` or `g`
property in the prototype file.
Given that proto.c only supports three numbers for permissions, we
need to remove the redundant information from the permission, else
it was incorrectly parsed.

Co-authored-by: Luca Di Maio <luca.dimaio1@gmail.com>
Co-authored-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Luca Di Maio <luca.dimaio1@gmail.com>
[aalbersh: 68 chars limit and removed patch review revisions]