f2fs: fix to account cp stats correctly

cp_foreground_calls sysfs entry shows total CP call count rather than
foreground CP call count, fix it.

Fixes: fc7100ea ("f2fs: Add f2fs stats to sysfs")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>