f2fs: add missing documentation for dir_level
f2fs: add missing documentation for dir_level

This patch adds missing dir_level documentation.

Signed-off-by: Jaegeuk Kim <20424effba9d4aeeb410130c5f23aa328fa29e09@samsung.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs