xprtrdma: Squelch a sparse warning

linux/include/trace/events/rpcrdma.h:501:1: warning: expression using sizeof bool
linux/include/trace/events/rpcrdma.h:501:1: warning: odd constant _Bool cast (ffffffffffffffff becomes 1)

Fixes: ab03eff5

 ("xprtrdma: Add trace points in RPC Call ... ")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>