(2.8r) Workaround for Issue 621

(2.8r) Workaround for Issue 621

Upon LUNetworkDisconnect() and LUNetworkConnect() try to acquire
all cluster's instances.

By that _LS_ACQUIRE_ALL acquire mode is set and not
_LS_ACQUIRE_EXACT and thus the deleted lock does cause any problem.

NOTE: This workaround is not merged upstream. They prefer to have
one opcode fail and make the end user retry, rather than have a
greedy way that locks all instances to resolve this. Since synnefo
does not use nodegroups, locking all instances is the same with
locking instances on default nodegroup.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>