f2fs: fix iostat parameter for discard

Just like other data we count uses the number of bytes as the basic unit,
but discard uses the number of cmds as the statistical unit. In fact the
discard command contains the number of blocks, so let's change to the
number of bytes as the base unit.

Fixes: b0af6d49 ("f2fs: add app/fs io stat")
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>