mm: fix the race between swapin_readahead and SWP_SYNCHRONOUS_IO path

The following race is observed due to which a processes faulting
on a swap entry, finds the page neither in swapcache nor swap. This
causes zram to give a zero filled page that gets mapped to the
process, resulting in a user space crash later.

Consider parent and child processes Pa and Pb sharing the same swap
slot with swap_count 2. Swap is on zram with SWP_SYNCHRONOUS_IO set.
Virtual address 'VA' of Pa and Pb points to the shared swap entry.

Pa                                       Pb

fault on VA                              fault on VA
do_swap_page                             do_swap_page
lookup_swap_cache fails                  lookup_swap_cache fails
                                         Pb scheduled out
swapin_readahead (deletes zram entry)
swap_free (makes swap_count 1)
                                         Pb scheduled in
                                         swap_readpage (swap_count == 1)
                                         Takes SWP_SYNCHRONOUS_IO path
                                         zram enrty absent
                                         zram gives a zero filled page

Fix this by reading the swap_count before lookup_swap_cache, which conforms
with the order in which page is added to swap cache and swap count is
decremented in do_swap_page. In the race case above, this will let Pb take
the readahead path and thus pick the proper page from swapcache.

Change-Id: I9a16078f514f1d51b5930e8cd74621128f4124d3
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>