UBI: fix oops in error path

This patch fixes an oops in the error path of 'ubi_attach_mtd_dev()'. If
anything after 'uif_init()' fails, we get an oops in 'cancel_pending()'. The
reason is that 'uif_close()' drops the last reference count for 'ubi->dev' and
whole 'struct ubi_device' is freed. And then
'ubi_wl_close()'->'cancel_pending()' tries to access the 'ubi' pointer and
problems begin.

Note, in 'ubi_detach_mtd_dev()' function we get a device reference to
work-around this issue. Do the same in the error path of
'ubi_attach_mtd_dev()'.
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/ubi/build.c

@@ -1019,6 +1019,8 @@ int ubi_attach_mtd_dev(struct mtd_info *mtd, int ubi_num, int vid_hdr_offset)
 out_debugfs:
 	ubi_debugfs_exit_dev(ubi);
 out_uif:
+	get_device(&ubi->dev);
+	ubi_assert(ref);
 	uif_close(ubi);
 out_detach:
 	ubi_wl_close(ubi);