rxrpc: Fix calc of resend age

[ Upstream commit 214a9dc7d852216e83acac7b75bc18f01ce184c2 ]

Fix the calculation of the resend age to add a microsecond value as
microseconds, not nanoseconds.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>