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>
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 832bd91..46311e7 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -130,6 +130,8 @@
 		need_cp = true;
 	else if (F2FS_I(inode)->xattr_ver == cur_cp_version(F2FS_CKPT(sbi)))
 		need_cp = true;
+	else if (test_opt(sbi, FASTBOOT))
+		need_cp = true;
 
 	return need_cp;
 }