Linux: Use kernel network helper functions

Linux: Use kernel network helper functions

Linux has various network helper functions available, which are
guaranteed to do the right thing in terms of setting address space
limits, not dereferencing NULL pointers, and the like. Use these, where
they're available, rather than rolling our own.

kernel_sendmsg and kernel_recvmsg appeared in 2.6.8.

Change-Id: I1cd91afd2182ad936756bbc4cf692262499c16e4
Reviewed-on: http://gerrit.openafs.org/1822
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>