s390x: lib: fix pgtable.h

Fix pgtable.h:

* SEGMENT_ENTRY_SFAA had one extra bit set
* pmd entries don't have a length field
* ipte does not need to clear the lower bits
 - clearing the 12 lower bits is technically incorrect, as page tables are
   architecturally aligned to 11 bit addresses (even though the unit tests
   allocate always one full page)
* region table entries should use REGION_ENTRY_TL instead of *_TABLE_LENGTH
 - *_TABLE_LENGTH need to stay, because they should be used for ASCEs

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Message-Id: <20210611140705.553307-4-imbrenda@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>