nfsd: Checking for acl support does not require fetching any acls

Whether or not a file system supports acls can be determined with
IS_POSIXACL(inode) and does not require trying to fetch any acls; the code for
computing the supported_attrs and aclsupport attributes can be simplified.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>