libzfs: zfs_userns: Don't leak the namespace fd

zfs_userns opens a file descriptor for the kernel to look up a
namespace, but does not close it.

Close the fd when we're done with it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #13554

lib/libzfs/os/linux/libzfs_util_os.c

@@ -274,5 +274,7 @@ zfs_userns(zfs_handle_t *zhp, const char *nspath, int attach)
 	if ((ret = zfs_ioctl(hdl, cmd, &zc)) != 0)
 		zfs_standard_error(hdl, errno, errbuf);
 
+	(void) close(zc.zc_cleanup_fd);
+
 	return (ret);
 }