Documentation: f2fs: Wrap snippets in literal code blocks

Documentation: f2fs: Wrap snippets in literal code blocks

Compression mode code and device aliasing shell snippets are shown
in htmldocs output as long-running paragraph instead. Wrap them.

Fixes: 602a16d58e9a ("f2fs: add compress_mode mount option")
Fixes: 128d333f0dff ("f2fs: introduce device aliasing file")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>