fd: filter anonymous FD from fd_lookup() result

Those callers whose requirements can accommodate an anonymous FD
need to call fd_anonymous(). fd_lookup() is strictly for open()ed
file descriptors only.

Change-Id: I5d8da3db0f44b2e2161d345d7a662cfd4ef87500
BUG: 846240
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4404
Tested-by: Gluster Build System <jenkins@build.gluster.com>