vfs_streams_xattr: fix check with samba_private_attr_name()

vfs_streams_xattr: fix check with samba_private_attr_name()

We want to check with samba_private_attr_name() whether the xattr name
is a private one, unfortunately it flags xattrs that begin with the
default streams prefix as private.  By only calling
samba_private_attr_name() in case the xattr does NOT begin with the
default prefix, we know that if it returns 'true' it definitely one of
our internal xattr like "user.DOSATTRIB".

This fixes a bug introduced in 634bcb09a08b927fd79ae0e16aeee2a123605f94
that denied all access to valid stream xattrs.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>