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

Authored by: Sowrabha Gopal <sowrabha.gopal@delphix.com>
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>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>

dir_is_empty_readdir() immediately returns if fdopendir() fails.
We should close dirfd when that happens.

OpenZFS-issue: https://www.illumos.org/issues/8430
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/e165e20
Closes #6289

lib/libzfs/libzfs_mount.c

@@ -210,6 +210,7 @@ dir_is_empty_readdir(const char *dirname)
 	}
 
 	if ((dirp = fdopendir(dirfd)) == NULL) {
+		(void) close(dirfd);
 		return (B_TRUE);
 	}