staging: erofs: make use of DBG_BUGON

DBG_BUGON is introduced and it could only crash when EROFS_FS_DEBUG
(EROFS developping feature) is on.
replace BUG_ON with DBG_BUGON.

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>