Reduce the checkpoint duration to 30secs

dirty_expire_centisecs is 30secs by default in Linux kernel. Let's use that
period to improve the durability in system wide.

Bug: 410100970
Bug: 288068123
Change-Id: I17c37b5ebcf44aa8feff81ba74fe2357a0ca2054
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>