nfs4acl: Fix owner mapping with ID_TYPE_BOTH

nfs4acl: Fix owner mapping with ID_TYPE_BOTH

This fixes a corner case when using NFS4 ACLs with ID_TYPE_BOTH.  Before
this patch, the owner entry in the ACL would be mapped to a gid entry in
the NFSv4 ACL, and not the expected special owner entry. This is caused
by the id mapping returning a valid gid and the nfs4 mapping assumed
that this was actually a group.

Fix this by asking for the uid first, and explicitly checking if the
mapped uid matches the owner. That creates a uid entry in the NFSv4 ACL
that can be changed later in smbacl4_substitute_{simple,special} to the
expected special owner entry.

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>