nilfs2: change default of 'errors' mount option to 'remount-ro' mode

Like ext3, nilfs has 'errors' mount option to allow specifying desired
behavior on severe errors.

Currently, the default action is 'errors=continue' and has potential
to advance filesystem corruption for severe errors.

This will change the action to 'errors=remount-ro' to avoid the issue.

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