f2fs: slightly reorganize read_raw_super_block

read_raw_super_block was introduced to help find the
first valid superblock. Commit da554e48

 ("f2fs:
recovering broken superblock during mount") changed the
behaviour to read both of them and check whether need
the recovery flag or not. So the comment before this
function isn't consistent with what it actually does.
Also, the origin code use two tags to round the err
cases, which isn't so readable. So this patch amend
the comment and slightly reorganize it.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>