arm64: standardize mmap_rnd() usage

In preparation for splitting out ET_DYN ASLR, this refactors the use of
mmap_rnd() to be used similarly to arm and x86.  This additionally
enables mmap ASLR on legacy mmap layouts, which appeared to be missing
on arm64, and was already supported on arm.  Additionally removes a
copy/pasted declaration of an unused function.

Change-Id: I85005b5a7ee8cbca019e5dad5fece4e8a194ae2d
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Git-commit: dd04cff1dceab18226853b555cf07914648a235f
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Se Wang (Patrick) Oh <sewango@codeaurora.org>