x86/selftests: Add test for mapping placement for 5-level paging

5-level paging provides a 56-bit virtual address space for user space
application. But the kernel defaults to mappings below the 47-bit address
space boundary, which is the upper bound for 4-level paging, unless an
application explicitely request it by using a mmap(2) address hint above
the 47-bit boundary. The kernel prevents mappings which spawn across the
47-bit boundary unless mmap(2) was invoked with MAP_FIXED.

Add a self-test that covers the corner cases of the interface and validates
the correctness of the implementation.

[ tglx: Massaged changelog once more ]

Signed-off-by: Kirill A. Shutemov <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: [email protected]
Cc: Linus Torvalds <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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