Update restrict.Restrict to both show the error message when failing to mount /dev/null over /proc/kcore, and to ignore "not exists" errors while doing so (for when CONFIG_PROC_KCORE=n in the kernel)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #5673 from tianon/kcore-error

Update restrict.Restrict to both show the error message when failing to mount /dev/null over /proc/kcore, and to ignore "not exists" errors while doing so (for when CONFIG_PROC_KCORE=n in the kernel)