nvmf/rdma: make disconnect qp from cm event safe.

The call to update_ibv_state could result in a segfault if the other
thread had already freed the qp and was just spinning on handling the
rdma event. By not updating the qpair state here, I don't think that we
lose any information about the qpair state. Especiallyy since just a
little bit later we update the qpair state to be in error.
There is nothing in the man pages about the cm events changing the ib
state although I imagine they are closely related. I just say that
because I believe that's why the update was originally in that spot.

fixes: GitHub issue #1110

Change-Id: I3f87ff009bc2019464ed7c6920dd71e2b286b3fd
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/477877


Community-CI: Broadcom SPDK FC-NVMe CI <spdk-ci.pdl@broadcom.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Alexey Marchuk <alexeymar@mellanox.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>