btrfs: merge alloc_device helpers

There are two helpers called in chain from one location, we can merge the
functionaliy.

Originally, alloc_fs_devices could fill the device uuid randomly if we
we didn't give the uuid buffer. This happens for seed devices but the
fsid is generated in btrfs_prepare_sprout, so we can remove it.

Signed-off-by: David Sterba <dsterba@suse.com>