Mount hook fails on sysfs-less containers

[allixx]

share/lxcfs/lxc.mount.hook fails to mount /sys/devices/system/cpu when target container doesn't have sysfs mounted, erroring the whole script due to #/bin/sh -e shebang, and preventing container start.

I propose making hook script more robust by adding a corresponding check.

https://github.com/allixx/lxcfs/commit/29ef6dce5fd5706d06a46f814ccfe04bde921abb