glfs: add writesame support
glfs: add writesame support

This will invoke zerro fill API due to there has no truely write
same routine yet. Since the writesame emulator in tcmur will do
the same, so it shouldn't be a problem of this.

Signed-off-by: Xiubo Li <d6cd9ec469dfb11098caa077ea0b7b3220fc70dc@redhat.com>

open-iscsi/tcmu-runner,open-iscsi/tcmu-runner,agrover/tcmu-runner