gluster: Change licence to GPLv2+

gluster: Change licence to GPLv2+

Pipe handling mechanism in gluster driver was based on similar implementation
in RBD driver and hence had GPLv2 and associated copyright information.
After changing gluster driver to coroutine based implementation, the pipe
handling code no longer exists and hence change gluster driver's licence to
GPLv2+ and remove RBD copyrights.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>