f2fs: introduce function read_raw_super_block()

Introduce function read_raw_super_block() to hide reading raw super block and
the retry routine if the first sb is invalid.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>