staging: erofs: fix the definition of DBG_BUGON

It's better not to positively BUG_ON the kernel, however developers
need a way to locate issues as soon as possible.

DBG_BUGON is introduced and it could only crash when EROFS_FS_DEBUG
(EROFS developping feature) is on. It is helpful for developers
to find and solve bugs quickly by eng builds.

Previously, DBG_BUGON is defined as ((void)0) if EROFS_FS_DEBUG is off,
but some unused variable warnings as follows could occur:

drivers/staging/erofs/unzip_vle.c: In function `init_alway:':
drivers/staging/erofs/unzip_vle.c:61:33: warning: unused variable `work' [-Wunused-variable]
  struct z_erofs_vle_work *const work =
                                 ^~~~

Fix it to #define DBG_BUGON(x) ((void)(x)).

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>