systemd/ceph-disk: reduce ceph-disk flock contention

systemd/ceph-disk: reduce ceph-disk flock contention

"ceph-disk trigger" invocation is currently performed in a mutually
exclusive fashion, with each call first taking an flock on the path
/var/lock/ceph-disk. On systems with a lot of osds, this leads to a
large amount of lock contention during boot-up, and can cause some
service instances to trip the 120 second timeout.

Take an flock on a device specific path instead of /var/lock/ceph-disk,
so that concurrent "ceph-disk trigger" invocations are permitted for
independent osds. This greatly reduces lock contention and consequently
the chance of service timeout. Per-device concurrency restrictions
required for http://tracker.ceph.com/issues/13160 are maintained.

Fixes: http://tracker.ceph.com/issues/18049
Signed-off-by: David Disseldorp <ddiss@suse.de>