ubifs: Fix error return code in alloc_wbufs()

Fix to return PTR_ERR() error code from the error handling case instead
fo 0 in function alloc_wbufs(), as done elsewhere in this function.

Fixes: 6a98bc46

 ("ubifs: Add authentication nodes to journal")
Signed-off-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>