staging: erofs: xattr.c: avoid BUG_ON

staging: erofs: xattr.c: avoid BUG_ON

Kill all the remaining BUG_ON in EROFS:
 - one BUG_ON was used to detect xattr on-disk corruption,
   proper error handling should be added for it instead;
 - the other BUG_ONs are used to detect potential issues,
   use DBG_BUGON only in (eng) debugging version.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20190813023054.73126-3-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>