svc: Change svc_sock_received to svc_xprt_received and export it

All fields touched by svc_sock_received are now transport independent.
Change it to use svc_xprt directly. This function is called from
transport dependent code, so export it.

Update the comment to clearly state the rules for calling this function.

Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
Acked-by: Neil Brown <neilb@suse.de>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Greg Banks <gnb@sgi.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>