SVCRDMA: Add check for XPT_CLOSE in svc_rdma_send

The svcrdma transport can crash if a send is waiting for an
empty SQ slot and the connection is closed due to an asynchronous error.
The crash is caused when svc_rdma_send attempts to send on a deleted
QP.
Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>