btrfs: merge alloc_dummy_extent_buffer() helpers

After previous patch removing nodesize from parameters,
__alloc_dummy_extent_buffer() and alloc_dummy_extent_buffer() are
identical so we can drop one.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>