erofs: get rid of a useless DBG_BUGON

`err` could be -EINTR and it should not be the case.  Actually such
DBG_BUGON is useless.

Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20230309053148.9223-2-hsiangkao@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>