erofs: add fscache context helper functions

Introduce a context structure for managing data blobs, and helper
functions for initializing and cleaning up this context structure.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220425122143.56815-13-jefflexu@linux.alibaba.com


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