zram: use generic start/end io accounting

zram: use generic start/end io accounting

Use bio generic_start_io_acct() and generic_end_io_acct() to account
device's block layer statistics.  This will let users to monitor zram
activities using sysstat and similar packages/tools.

Apart from the usual per-stat sysfs attr, zram IO stats are now also
available in '/sys/block/zram<id>/stat' and '/proc/diskstats' files.

We will slowly get rid of per-stat sysfs files.

Signed-off-by: Sergey Senozhatsky <[email protected]>
Acked-by: Minchan Kim <[email protected]>
Cc: Nitin Gupta <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>