LU-15853 tests: sanity 104c should use $MOUNT

LU-15853 tests: sanity 104c should use $MOUNT

Replace the hardcoded mount point value in test sanity 104c
with '$MOUNT'.

Test-Parameters: trivial testlist=sanity env=ONLY=104
Fixes: 8ee6e1c ("LU-14565 ofd: Do not rely on tgd_blockbit")
Change-Id: Iead6792115bd92eb53e3da55040694feafcced8d
Signed-off-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-on: https://review.whamcloud.com/47342
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>