f2fs: fix lost xattrs of directories

This patch enhances the xattr consistency of dirs from suddern power-cuts.

Possible scenario would be:
1. dir->setxattr used by per-file encryption
2. file->setxattr goes into inline_xattr
3. file->fsync

In that case, we should do checkpoint for #1.
Otherwise we'd lose dir's key information for the file given #2.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>