Reset sx_fbtbc to 0 once we are sure it's a haproxy v2 header fragment

When receiving a haproxy v2 message, after processing the first fragment
which only contain the proxy header, the ::recv might return EGAIN if the
next fragment is not ready to read yet, then the function svc_vc_recv
would return, but left xd->sx_fbtbx == 0x0d0a0d0a (PP2_SIG_UINT32), which
was set when parsing the haproxy v2 header. When the next fragment is ready,
the epoll event for the same xprt will be trigged, then a non-zero
xd->sx_fbtbc would deceit sv_vc_recv into derefrencing a NULL uv pointer.

Stack trace:
Thread 76 "ganesha.nfsd" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fc9937ee640 (LWP 2931)]
0x00007fc9f3e30fa6 in svc_vc_recv (xprt=0x7fc9dc00e3b0) at /git/nfs-ganesha/src/libntirpc/src/svc_vc.c:1070
1070			flags = uv->u.uio_flags;
(gdb) bt
(gdb) p uv
$1 = (struct xdr_ioq_uv *) 0x0
(gdb) p xd->sx_fbtbc
$2 = 218762506

src/svc_vc.c

@@ -775,6 +775,8 @@ static enum haproxy_ret_code handle_haproxy_header(SVCXPRT *xprt)
 	struct proxy_header_part s;
 	union proxy_addr pa;
 	enum haproxy_ret_code ret;
+	struct rpc_dplx_rec *rec = REC_XPRT(xprt);
+	struct svc_vc_xprt *xd = VC_DR(rec);
 
 	__warnx(TIRPC_DEBUG_FLAG_SVC_VC,
 			"%s: %p fd %d potential haproxy packet",
@@ -803,6 +805,8 @@ static enum haproxy_ret_code handle_haproxy_header(SVCXPRT *xprt)
 		return HAPROXY_RET_CODE__NOT_HAPROXY;
 	}
 
+	/* Reset the sx_fbtbc for HA Proxy v2*/
+	xd->sx_fbtbc = 0;
 	rlen = recv(xprt->xp_fd, rest, sizeof(rest), MSG_WAITALL);
 	if (rlen != sizeof(rest)) {
 		__warnx(TIRPC_DEBUG_FLAG_ERROR,