lvmlockd: use lvmlock LV size

lvmlockd: use lvmlock LV size

Previously, lvmlockd detected the end of the lvmlock LV
by doing i/o to it until an i/o error was returned.
This triggered sanlock warning messages, so use the LV
size to avoid accessing beyond the end of the device.

Previously, every lvcreate would refresh the lvmlock LV
in case another machine had extended it.  This involves
a lot of unnecessary work in most cases, so now compare
the LV size and device size to detect when a refresh is
needed.