rxrpc: Add ktime_sub_ms()

Add a ktime_sub_ms() to go with ktime_add_ms() and co. for use in AF_RXRPC
RTT determination.
Signed-off-by: David Howells <dhowells@redhat.com>

include/linux/ktime.h

@@ -231,6 +231,11 @@ static inline ktime_t ktime_sub_us(const ktime_t kt, const u64 usec)
 	return ktime_sub_ns(kt, usec * NSEC_PER_USEC);
 }
 
+static inline ktime_t ktime_sub_ms(const ktime_t kt, const u64 msec)
+{
+	return ktime_sub_ns(kt, msec * NSEC_PER_MSEC);
+}
+
 extern ktime_t ktime_add_safe(const ktime_t lhs, const ktime_t rhs);
 
 /**