erofs: drop experimental warning for FSDAX

As EXT4/XFS filesystems, FSDAX functionality is considered to be stable.
Let's drop this warning.

Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240325005116.106351-1-hsiangkao@linux.alibaba.com