rxrpc: Fix a warning

Fix a warning due to an uninitialised variable.

le included from ../fs/afs/fs_probe.c:11:
../fs/afs/fs_probe.c: In function 'afs_fileserver_probe_result':
../fs/afs/internal.h:1453:2: warning: 'rtt_us' may be used uninitialized in this function [-Wmaybe-uninitialized]
 1453 |  printk("[%-6.6s] "FMT"\n", current->comm ,##__VA_ARGS__)
      |  ^~~~~~
../fs/afs/fs_probe.c:35:15: note: 'rtt_us' was declared here

Signed-off-by: David Howells <dhowells@redhat.com>