x86/mm: Introduce arch_rnd() to compute 32/64 mmap random base

The compat (32bit) mmap() sycall issued by a 64-bit task results in a
mapping above 4GB. That's outside the compat mode address space and
prevents CRIU to restore 32bit processes from a 64bit application.

As a first step to address this, split out the address base randomizing
calculation from arch_mmap_rnd() into a helper function, which can be used
independent of mmap_ia32() based decisions.

[ tglx: Massaged changelog ]

Suggested-by: Thomas Gleixner <[email protected]>
Signed-off-by: Dmitry Safonov <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: Andy Lutomirski <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>

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