nilfs2: fix irregular checkpoint creation due to data flush

When nilfs flushes out dirty data to reduce memory pressure, creation
of checkpoints is wrongly postponed.  This bug causes irregular
checkpoint creation especially in small footprint systems.

To correct this issue, a timer for the checkpoint creation has to be
continued if a log writer does not create a checkpoint.

This will do the correction.

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