xprtrdma: checking for NULL instead of IS_ERR()

The rpcrdma_create_req() function returns error pointers or success.  It
never returns NULL.

Fixes: f531a5db ('xprtrdma: Pre-allocate backward rpc_rqst and send/receive buffers')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>