performance/readdir-ahead: indicate EOF for readdirp

posix xlator sends op_errno as ENOENT and op_ret as 0, to indicate readdir has
been completed. readdir-ahead should send that op_errno that it has saved in the
fd context, when it serves the readdir requests. Otherwise some xlators sitting
above performance xlators such as snapview-client, which checks for end of
readdir operation by checking op_ret to 0 and op_errno to ENOENT will not be
able to identify end of readdir.

Change-Id: Ib0835136c61cb1e0d7df933226c479c7db703a71
BUG: 1174783
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/9283
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>