net: packet: make pkt_sk() inline

It's better make 'pkt_sk()' inline here, as non-inline function
shouldn't occur in headers. Besides, this function is simple
enough to be inline.

Signed-off-by: Menglong Dong <dong.menglong@zte.com.cn>
Link: https://lore.kernel.org/r/20210127123302.29842-1-dong.menglong@zte.com.cn


Signed-off-by: Jakub Kicinski <kuba@kernel.org>