8430 dir_is_empty_readdir() doesn't properly handle error from fdopendir()

illumos/illumos-gate@ba6e7e6505150388de6dc6a88741164118a421bf
https://github.com/illumos/illumos-gate/commit/ba6e7e6505150388de6dc6a88741164118a421bf

https://www.illumos.org/issues/8430

we should close dirfd if fdopendir() fails.

Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Sowrabha Gopal <sowrabha.gopal@delphix.com>