ubifs: Reduce kfree() calls in ubifs_purge_xattrs()

Move a pair of kfree() calls behind the label “out_err”
so that two statements can be better reused at the end of
this function implementation.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>