storage/posix: Prefer gfid links for inode-handle

Problem:
File path could change by other entry operations in-flight so if renames are in
progress at the time of other operations like open, it may lead to failures.
We observed that this issue can also happen while renames and readdirps/lookups
are in progress because dentry-table is going stale sometimes.

Fix:
Prefer gfid-handles over paths for files. For directory handles prefering
gfid-handles hits performance issues because it needs to resolve paths
traversing up the symlinks.
Tests which test if files are opened should check on gfid path after this change.
So changed couple of tests to reflect the same.

Note:
This patch doesn't fix the issue for directories. I think a complete fix is to
come up with an entry operation serialization xlator. Until then lets live with
this.

Change-Id: I10bda1083036d013f3a12588db7a71039d9da6c3
BUG: 1136159
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/8575
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>