scripts/setup: Fix check for mounted devices

The previous approach didn't take into the account systems where root
partition is representend in a virtual form of /dev/root (see
clearlinux). In case the device was bound to virtio this would fool
setup.sh into thinking nothing is mounted and proceed with unbinding
the entire rootfs.

Fix this by checking the maj:min numbers of the device instead of its
name against the mountinfo list.

Change-Id: I0feb2584869f6bb72df3e9a4e619620240cfce3b
Signed-off-by: Michal Berger <michalx.berger@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/4046


Reviewed-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-by: Maciej Wawryk <maciejx.wawryk@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>