mm: swap: swap ratio support

Add support to receive a static ratio from userspace to
divide the swap pages between ZRAM and disk based swap
devices. The existing infrastructure allows to keep
same priority for multiple swap devices, which results
in round robin distribution of pages. With this patch,
the ratio can be defined.

Change-Id: I54f54489db84cabb206569dd62d61a8a7a898991
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>
[swatsrid@codeaurora.org: Fix trivial merge conflicts]
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>