mount.nilfs2: invoke cleanerd even if no-mtab option is specified

After commit 7f7b227f8a8eda5603b58899597c2fdee2a73ab7 "libmount: don't
base GC startup on no-mtab context" was applied, behavior of the new
mount.nilfs2 (libmount based implementation) was slightly changed.  It
now invokes cleanerd even when no-mtab option "-n" is specified.

This patch makes a similiar change to the legacy mount.nilfs2, and
updates the manpage of mount.nilfs2 to reflect the change.

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