Revert "f2fs: use percpu_counter for # of dirty pages in inode"

This reverts commit 1beba1b3

.

The perpcu_counter doesn't provide atomicity in single core and consume more
DRAM. That incurs fs_mark test failure due to ENOMEM.

Cc: stable@vger.kernel.org # 4.7+
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>