os/bluestore/KernelDevice: use flock(2) for block device lock

os/bluestore/KernelDevice: use flock(2) for block device lock

The fcntl locks fail due to the classic posix lock gotcha: if you close
*any* fd to the same inode from the process, the lock(s) go away.

Use flock(2) instead.  We have to be careful because we open the main
bluestore device via two KernelDevice instances: one for bluestore and
one for bluefs.  Add a no-lock flag so that the bluefs instance does not
try to lock and does not conflict with bluestore's.

Fixes: http://tracker.ceph.com/issues/38150
Signed-off-by: Sage Weil <sage@redhat.com>