bash-completion: Use default completion if there are no matches

This helps in completing, for instance, file paths as arguments to
some of the rpc functions (like bdev_aio_create, etc.).

Signed-off-by: Michal Berger <michalx.berger@intel.com>
Change-Id: Id0880773fade3fc6128ac9de6abfe19f6b40077f
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/8447


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-by: Pawel Piatek <pawelx.piatek@intel.com>