nvmf/rpc: Put channel refcount after nvmf_subsystem_get_qpairs use

nvmf_subsystem_get_qpairs RPC handler may cause the program
could not exit normally (e.g. ctrl c). Reason is that,
spdk_get_io_channel() will be called during the getting
qpairs stream, which will add refcount value for each
existing channel. When end target, channel cannot be
destroyed since refcount be added additionally and
its value could not be subtracted to 0. As a result,
the program will hang in the process of exiting.

So here we don't need to allocate a new channel, just use
the exist one.

Signed-off-by: zhaoshushu.zss <zhaoshushu.zss@alibaba-inc.com>
Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
Change-Id: I08b4678edaa9404b8e8af125ebae572b31edf77e
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/9881


Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>
Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>