ceph-disk: use unix lock instead of lockfile class

The lockfile class relies on file system trickery to get safe mutual
exclusion.  However, the unix syscalls do this for us.  More
importantly, the unix locks go away when the owning process dies, which
is behavior that we want here.

Fixes: #5387
Signed-off-by: Sage Weil <sage@inktank.com>