erofs: enable long extended attribute name prefixes

Let's enable long xattr name prefix feature.  Old kernels will just
ignore / skip such extended attributes.  In addition, in case you
don't want to mount such images, add another incompatible feature as
an option for this.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Acked-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20230407222808.19670-1-jefflexu@linux.alibaba.com


[ Gao Xiang: minor commit message fix. ]
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>