LU-5499 tests: use grep -w to search /proc/mounts

LU-5499 tests: use grep -w to search /proc/mounts

When searching for /sbin/mount.lustre in /proc/mounts, use "grep -qw"
instead of using a trailing space, because if the mount.lustre binary
is deleted while it is mounted (e.g. by "make clean") it may have a
non-printable character following it and not be found and unmounted.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: Ia33c4d4b7efa73f543999f73da198fa0698cab07
Reviewed-on: http://review.whamcloud.com/12409
Tested-by: Jenkins
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>