Btrfs-progs: add missing write check for mkfs

Btrfs-progs: add missing write check for mkfs

Assert that the writes of the device and chunk tree
roots succeed. This verification is currently done
for all other tree roots, however it was missing for
those 2 trees.

Would these tree root writes fail, but all others succeed,
it would lead to a corrupted/incomplete btrfs filesystem,
or, more likely some weird failure later on in mkfs.btrfs
inside open_ctree().

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>