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/f2fs.h b/fs/f2fs/f2fs.h
index 5a97058..d45f3f4 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -49,6 +49,7 @@
 #define F2FS_MOUNT_INLINE_DENTRY	0x00000200
 #define F2FS_MOUNT_FLUSH_MERGE		0x00000400
 #define F2FS_MOUNT_NOBARRIER		0x00000800
+#define F2FS_MOUNT_FASTBOOT		0x00001000
 
 #define clear_opt(sbi, option)	(sbi->mount_opt.opt &= ~F2FS_MOUNT_##option)
 #define set_opt(sbi, option)	(sbi->mount_opt.opt |= F2FS_MOUNT_##option)