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

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

Some distros may link /etc/mtab to /proc/mounts, or
/proc/self/mounts.  In such case, we don't need to
update mtab. Otherwise we false alart user with
"
mount.lustre: addmntent: Invalid argument:
"

Signed-off-by: Peng Tao <tao.peng@emc.com>
Change-Id: I2e3e213e4ee3bc177865c4ca7435a7eecd274b46
Reviewed-on: http://review.whamcloud.com/3881
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>