nilfs2: add nodiscard mount option

Nilfs has "discard" mount option which issues discard/TRIM commands to
underlying block device, but it lacks a complementary option and has
no way to disable the feature through remount.

This adds "nodiscard" option to resolve this imbalance.

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

diff --cc Documentation/filesystems/nilfs2.txt
Simple merge

diff --cc fs/nilfs2/super.c
Simple merge