erofs: decouple basic mount options from fs_context

Previously, EROFS mount options are all in the basic types, so
erofs_fs_context can be directly copied with assignment. However,
when the multiple device feature is introduced, it's hard to handle
multiple device information like the other basic mount options.

Let's separate basic mount option usage from fs_context, thus
multiple device information can be handled gracefully then.

No logic changes.

Link: https://lore.kernel.org/r/20211007070224.12833-1-hsiangkao@linux.alibaba.com
Reviewed-by: Chao Yu <chao@kernel.org>
Reviewed-by: Liu Bo <bo.liu@linux.alibaba.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>