sunrpc: Fix return value of get_srcport()

Since bc1c56e9 transport->srcport may by unset, causing
get_srcport() to return 0 when called. Fix this by querying the port
from the underlying socket instead of the transport.

Fixes: bc1c56e9

 (SUNRPC: prevent port reuse on transports which don't request it)
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>