sparc64: Clean up 64-bit mmap exclusion defines.

Older UltraSPARC chips had an address space hole due to the MMU only
supporting 44-bit virtual addresses.

The top end of this hole also has the same value as the current
definition of PAGE_OFFSET, so this can be confusing.

Consolidate the defines for the userspace mmap exclusion range into
page_64.h and use them in sys_sparc_64.c and hugetlbpage.c

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Bob Picco <bob.picco@oracle.com>