commits as of 04/16/2014

f2fs: flush dirty directory pages when scattered pages are gathered
f2fs: fix to decrease the number of dirty dentry page
f2fs: remove costly dirty_dir_inode operations
f2fs: avoid to conduct roll-forward due to the remained garbage blocks
f2fs: enable flush_merge only in f2fs is not read-only
f2fs: use __GFP_ZERO to avoid appending set-NULL
f2fs: put the bio when issue_flush completed