Btrfs-progs: remove unused variable and update btrfs-image man page

Btrfs-progs: remove unused variable and update btrfs-image man page

Remove unused variable in btrfs-image.c (update_super) and update man
page documentation about -r option. Running btrfsck on a restored
image produces missing chunk information. This is because by default,
btrfs-image fixes up chunk tree to use 1 stripe pointing to the
primary device. This in turns results in btrfsck making some noise.

$  ./mkfs.btrfs /dev/sdb2 -f
$  ./btrfs-image /dev/sdb2 btrfs_image_output
$  ./btrfs-image -r btrfs_image_output disk-image
$  ./btrfsck disk-image

Device extent[1, 29360128, 8388608] didn't find the relative chunk.
Device extent[1, 1111490560, 1073741824] didn't find the relative chunk.

Ideally btfsck should be updated to reflect this default behavior and
not through these messages, but it isn't harmful and can be done
later.

Signed-off-by: Rakesh Pandit <rakesh@tuxera.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>