s390/mm: fix off-by-one bug in 5-level page table handling

Martin Cermak reported that setting a uprobe doesn't work. Reason for
this is that the common uprobes code tries to get an unmapped area at
the last possible page within an address space.

This broke with commit 1aea9b3f9210 ("s390/mm: implement 5 level pages
tables") which introduced an off-by-one bug which prevents to map
anything at the last possible page within an address space.

The check with the off-by-one bug however can be removed since with
commit 8ab867cb0806 ("s390/mm: fix BUG_ON in crst_table_upgrade") the
necessary check is done at both call sites.

Reported-by: Martin Cermak <mcermak@redhat.com>
Bisected-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Fixes: 1aea9b3f9210 ("s390/mm: implement 5 level pages tables")
Cc: <stable@vger.kernel.org> # v4.13+
Reviewed-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/pgalloc.c

@@ -83,8 +83,6 @@ int crst_table_upgrade(struct mm_struct *mm, unsigned long end)
 
 	/* upgrade should only happen from 3 to 4, 3 to 5, or 4 to 5 levels */
 	VM_BUG_ON(mm->context.asce_limit < _REGION2_SIZE);
-	if (end >= TASK_SIZE_MAX)
-		return -ENOMEM;
 	rc = 0;
 	notify = 0;
 	while (mm->context.asce_limit < end) {