Linux sandbox: handle ENOSPC clone failure

Linux 4.9 adds /proc/sys/user/max_user_namespaces to limit the number
of concurrent user namespaces.  Sometimes the limit is set to 0 in
which case clone() will return ENOSPC.  This CL avoids PCHECK()ing in
that case.

BUG=715138
R=mdempsky@chromium.org

Review-Url: https://codereview.chromium.org/2842033002
Cr-Commit-Position: refs/heads/master@{#467271}
1 file changed