f2fs: remove costly dirty_dir_inode operations

This patch removes list opeations in handling dirty dir inodes.
Previously, F2FS traverses whole the list of dirty dir inodes to check
whether
there is an existing inode or not, resulting in heavy CPU overheads.

So this patch removes such the traverse operations by adding
FI_DIRTY_DIR to
indicate the inode lies on the list or not.
Through this simple flag, we can remove redundant operations gracefully.