LU-1833 util: don't update mtab if linked to /proc

LU-1833 util: don't update mtab if linked to /proc

Backport patch of LU-1833 for b2_1.
This is needed for the diskless environment since their
mtab is linking to /proc/mount.

Signed-off-by: Shuichi Ihara <sihara@ddn.com>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Change-Id: Id18cd6a820dff905f0d61ecc1f71f4ce7b48cf77
Reviewed-on: http://review.whamcloud.com/4785
Tested-by: Hudson
Reviewed-by: Peng Tao <bergwolf@gmail.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>