storage/posix: Fix race in file creation when brick was offline during delete.

Summary:
If a file is deleted while a brick is offline, and a file by the same
name is subsequently created once the brick has been brought back
online but not fully healed, the brick returns the old, pre-deletion
GFID for the file, This results in lack of consensus about the GFID,
which is bad, and usually manifests as EROFS.

Fix by making bricks honor the GFID requested by the server when
a file is created, even if there is a pre-existing one. This fixes
the race and has the additional benefit that any clients with the old
GFID will get ESTALE accessing the old file instead of silently
getting access to a file which differs fromthe one they thought they
were accessing.

- This is a port of D3122637 to 3.8.

Test Plan:
- 12 hours of untarring linux source tarball onto 2-way replicated
volume while bringing the second replica server up and down every
30 seconds. Previously consistently failed with EROFS within half
an hour. Logs show that new code is being invoked. Mass MD5 over
resulting directory shows that all files were correctly untarr'd
and are readable.

- Prove tests

Signed-off-by: Shreyas Siravara <sshreyas>

Change-Id: I069e55a5bdfc8de81b1602a093d36fa82f38f9cd
Reviewed-on: https://review.gluster.org/16339
Reviewed-by: Kevin Vigor <kvigor>
Smoke: Gluster Build System <jenkins.org>
Tested-by: Shreyas Siravara <sshreyas>
CentOS-regression: Gluster Build System <jenkins.org>
NetBSD-regression: NetBSD Build System <jenkins.org>