Functions added in the 2.6.22 kernel in tcp.h and skbuff.h
have been added to the compatibility headers. Additionally
the data argument was dropped in 2.6.22 for the work queue
macros, requiring them to be redfined for previous kernels.
Based on changes from David Erickson.
NLMSG_DEFAULT_SIZE was introduced in 2.6.20-rc1 in commit 339bf98ff,
"[NETLINK]: Do precise netlink message allocations where possible", but
we only defined it for kernels 2.6.18 and earlier. This change fixes
up the discrepancy.