cluster/afr: Perform gfid heal inside locks.

        Backport of http://review.gluster.org/8512

Problem:
Allowing lookup with 'gfid-req' will lead to assigning gfid at posix layer.
When two mounts perform lookup in parallel that can lead to both bricks getting
different gfids leading to gfid-mismatch/EIO for the lookup.

Fix:
Perform gfid heal inside lock.

BUG: 1136823
Change-Id: I1059fcd38338348b7e96a0bd4c35b0f49bf77aec
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/8589
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>