svc: Make the enqueue service transport neutral and export it.

The svc_sock_enqueue function is now transport independent since all of
the fields it touches have been moved to the transport independent svc_xprt
structure. Change the function to use the svc_xprt structure directly
instead of the transport specific svc_sock structure.

Transport specific data-ready handlers need to call this function, so
export it.

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>