rxrpc: Make rxrpc_kernel_get_srtt() indicate validity

Fix rxrpc_kernel_get_srtt() to indicate the validity of the returned
smoothed RTT.  If we haven't had any valid samples yet, the SRTT isn't
useful.

Fixes: c410bf01 ("rxrpc: Fix the excessive initial retransmission timeout")
Signed-off-by: David Howells <dhowells@redhat.com>