LinuxKPI: skbuff: remove _alloc_len field

Initially we saved the exact allocation length for contigfree(9).
contigfree can now be replaced by free(9) so there is no need to
remember that value anymore. Removing it also simplifies the
linuxkpi_kfree_skb() code.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days