Synchronize extend_volume methods in 3PAR drivers

The extend_volume methods in the HP3PAR FC and
iSCSI drivers should have been synchronized with
the synchronize annotation, but were not.

Change-Id: Ic7f18b7314e10dfb499b16f43b72367fe3ed1176
Closes-Bug: #1220947
Merge "Synchronize extend_volume methods in 3PAR drivers"