posix: remove duplicate dirfd calls in posix_opendir

BUG: 1168910
Change-Id: I285d352d20374bb3edee2db42d062d4724198425
Signed-off-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-on: http://review.gluster.org/9186
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>