skbuff: Add likely to skb pointer in build_skb()

Similarly to napi_build_skb(), it is likely the skb allocation in
build_skb() succeeded. frag_size != 0 is also likely, as stated in
__build_skb_around().

Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Larysa Zaremba <larysa.zaremba@intel.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>