mm: swap: fix swapcache usage for fast swap devices

1) The swap readahead algorithm need not be applied for fast swap
devices like zram.
2) Code to set SWP_FAST is placed incorrectly, resulting in the flag
not being set.
Fix these to reduce the swapcache usage.

Change-Id: I23d9af5819f4b25f90f14a12657fa19ed401fb2a
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>