f2fs: save device node number into f2fs_inode

f2fs: save device node number into f2fs_inode

This patch stores inode->i_rdev into on-disk inode structure.

Alun reported that:
 aspire tmp # mount -t f2fs /dev/sdb mnt
 aspire tmp # mknod mnt/sda1 b 8 1
 aspire tmp # mknod mnt/null c 1 3
 aspire tmp # mknod mnt/console c 5 1
 aspire tmp # ls -l mnt
 total 2
 crw-r--r-- 1 root root 5, 1 Jan 22 18:44 console
 crw-r--r-- 1 root root 1, 3 Jan 22 18:44 null
 brw-r--r-- 1 root root 8, 1 Jan 22 18:44 sda1
 aspire tmp # umount mnt
 aspire tmp # mount -t f2fs /dev/sdb mnt
 aspire tmp # ls -l mnt
 total 2
 crw-r--r-- 1 root root 0, 0 Jan 22 18:44 console
 crw-r--r-- 1 root root 0, 0 Jan 22 18:44 null
 brw-r--r-- 1 root root 0, 0 Jan 22 18:44 sda1

In this report, f2fs lost the major/minor numbers of device files after umount.
The reason was revealed that f2fs does not store the inode->i_rdev to the
on-disk inode data structure.

So, as the other file systems do, f2fs also stores i_rdev into the i_addr fields
in on-disk inode structure without any on-disk layout changes.
Note that, this bug is limited to device files made by mknod().

Reported-and-Tested-by: Alun Jones <alun.linux@ty-penguin.org.uk>
Signed-off-by: Changman Lee <cm224.lee@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>