cluster/afr: unlock before aborting transaction

Else this results in a missing frame causing a hang

Change-Id: Ib5f3dc6a3999449faa2853cee2944af2fb065a20
BUG: 1002399
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5878
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>