zram: export new 'io_stat' sysfs attrs

zram: export new 'io_stat' sysfs attrs

Per-device `zram<id>/io_stat' file provides accumulated I/O statistics of
particular zram device in a format similar to block layer statistics.  The
file consists of a single line and represents the following stats
(separated by whitespace):

        failed_reads
        failed_writes
        invalid_io
        notify_free

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]>