erofs: register fscache context for extra data blobs

Similar to the multi-device mode, erofs could be mounted from one
primary data blob (mandatory) and multiple extra data blobs (optional).

Register fscache context for each extra data blob.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220425122143.56815-17-jefflexu@linux.alibaba.com


Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>