btrfs-progs: fix BUG_ON when all devices under seed fs are missing

btrfs-progs: fix BUG_ON when all devices under seed fs are missing

Steps to reproduce:
	# mkfs.btrfs -f /dev/sda[1-2]
	# btrfstune -S 1 /dev/sda1
	# mount /dev/sda /mnt
	# btrfs dev add /dev/sda3 /mnt
	# umount /mnt
	# mkfs.ext4 /dev/sda1		// kill seed dev
	# mkfs.ext4 /dev/sda2		// kill seed dev
	# btrfs-debug-tree /dev/sda3	<== BUG_ON
Output msg:
	volumes.c:1824: btrfs_read_chunk_tree: Assertion `ret` failed.
	btrfs-debug-tree[0x41cb36]
	btrfs-debug-tree(btrfs_read_chunk_tree+0x3ca)
	btrfs-debug-tree(btrfs_setup_chunk_tree_and_device_map
	btrfs-debug-tree[0x40f695]
	btrfs-debug-tree(open_ctree_fs_info+0x86)
	btrfs-debug-tree(main+0x12d)
	/lib64/libc.so.6(__libc_start_main+0xf5)
	btrfs-debug-tree[0x4062e9]

This BUG_ON complains about a failed @read_one_dev() call when
@open_seed_devices() failed to find the seed @fs_devices object
for a dev_item in chunk tree.
In this case, just insert a "shadow" @fs_devices with the fsid in
dev_item shall make no harm since no other tools will try to
make use of the stuff that the "shadow" @fs_devices possesses
after its creation.

After apply this commit, btrfs-debug-tree will report unable
to open the device.

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>