nilfs2: implement fallback for super root search

Although nilfs redundantly uses two super blocks and each may point to
different position on log, the current version of nilfs does not try
fallback to the spare super block when it doesn't find any valid log
at the position that the primary super block points to.

This has been a cause of mount failures due to write order reversals
on barrier less block devices.

This inserts fallback code in error path of nilfs_search_super_root
routine to resolve the mount failure problem.

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