LU-4423 utils: Remove variables qualified by 'extern' under mount_utils.c

LU-4423 utils: Remove variables qualified by 'extern' under mount_utils.c

Removal of corresponding 'extern' declaration is already done
under commit b43c7cc6cba514687ebcbeaa1a7dcc8cf7ffa694.

This patch is an extension to the above commit and covers
file mount_utils.c

Test-Parameters: trivial
Signed-off-by: Arshad Hussain <arshad.super@gmail.com>
Change-Id: I2362be0f7ec32fba9d9e680a3d7c416e5ec40083
Reviewed-on: https://review.whamcloud.com/35688
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>