xprtrdma: Cancel refresh worker during buffer shutdown

Trying to create MRs while the transport is being torn down can
cause a crash.

Fixes: e2ac236c ("xprtrdma: Allocate MRs on demand")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>