nilfs2: refactor recovery logic routines

Most functions in recovery code take an argument of a super block
instance or a nilfs_sb_info struct for convenience sake.

This replaces them aggressively with a nilfs object by applying
__bread and __breadahead against routines using sb_bread and
sb_breadahead.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>