only set the PRIV_FLAGS_NEEDS_FLUSH flag on PMEM buffers

PRIV_FLAGS_NEEDS_FLUSH will trigger an ioctl call into the PMEM
driver in unlock(), so it makes not sense to set this flag for
non pmem buffers.

Change-Id: Ie1e077c10df86d3689b82fa6ce5d6c856fc95688
1 file changed