nilfs2: add barrier mount option

Nilfs enables write barriers by default and has "nobarrier" mount
option to disable this feature.  But it lacks the complementary option
and has no way to re-enable the feature on remount.

This adds "barrier" option to resolve this imbalance.

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