f2fs: introduce -o fastboot for reducing booting time only

If a system wants to reduce the booting time as a top priority, now we can
use a mount option, -o fastboot.
With this option, f2fs conducts a little bit slow write_checkpoint, but
it can avoid the node page reads during the next mount time.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>