s390/mm: change swap pte encoding and pgtable cleanup

After the file ptes have been removed the bit combination used to
encode non-linear mappings can be reused for the swap ptes. This
frees up a precious pte software bit. Reflect the change in the
swap encoding in the comments and do some cleanup while we are
at it.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

diff --cc arch/s390/include/asm/pgtable.h
Simple merge

diff --cc arch/s390/mm/hugetlbpage.c
Simple merge