afr: make NOP truncate/ftruncate efficient

If truncate/ftruncate is called with the offset as the current size
of file, then skip the durability fsync and unwind quickly.

Change-Id: I0baec68d96c6d4d8217d33bd9738f7ed0d1b40c5
BUG: 958118
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5737
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>