From: Martin Schwidefsky <schwidefsky@de.ibm.com>

This fixes a problem in sys_swapon that can cause the creation of invalid
swap ptes.  This has its cause in the arch-independent swap entries vs. 
the pte coded swap entries.  The swp_entry_t uses 27 bits for the offset
and 5 bits for the type.  In sys_swapon this definition is used to find how
many swap devices and how many pages on each device there can be.  But the
swap entries encoded in a pte can be subject to additional restrictions due
to the hardware besides the 27/5 division of the bits in the swp_entry_t
type.  This is solved by adding pte_to_swp_entry and swp_entry_to_pte calls
to the calculations for maximum type and offset.

In addition the s390 swap pte division for offset/type is changed from 19/6
bits to 20/5 bits.