mm: allow for outstanding swap writeback accounting

To prevent flooding the swap device with writebacks, frontswap
backends need to count and limit the number of outstanding
writebacks.  The incrementing of the counter can be done before
the call to __swap_writepage().  However, the caller must receive
a notification when the writeback completes in order to decrement
the counter.

To achieve this functionality, this patch modifies
__swap_writepage() to take the bio completion callback function
as an argument.

end_swap_bio_write(), the normal bio completion function, is also
made non-static so that code doing the accounting can call it
after the accounting is done.

Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Acked-by: Minchan Kim <minchan@kernel.org>