f2fs: introduce get_dirty_dents for readability

The get_dirty_dents gives us the number of dirty dentry pages.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

diff --cc fs/f2fs/checkpoint.c
Simple merge

diff --cc fs/f2fs/f2fs.h
Simple merge

diff --cc fs/f2fs/inode.c
Simple merge