mips: extract logic for mmap_rnd()

In preparation for splitting out ET_DYN ASLR, extract the mmap ASLR
selection into a separate function.

Change-Id: I9851bf765016d7eb1f93a12b61ca9779b482abba
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Git-commit: 1f0569df
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


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