s390/tlb: Use mm_has_pgste() instead of mm_alloc_pgste()

An mm has pgstes only after s390_enable_sie() has been called, while
mm_alloc_pgste() may be always true (e.g. via sysctl setting).

Limit the calls to gmap_unlink() in pte_free_tlb() to those cases
where there might be something to unlink.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>