test/blobfs: Fix typo in the mount check

The actual check was ORing the execution of mount instead of sending
its stdout to grep - in case mount returned with != 0, this would
block execution of the entire script as grep would wait for input on
its stdin still attached to a terminal.

Also, quote $mount_dir to avoid potential word splitting.

Change-Id: Iea531eef0af79a3742e0a954ea6188e304d0d14a
Signed-off-by: Michal Berger <michallinuxstuff@gmail.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/483433


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>