iomap: add caller-provided callbacks for read and readahead

iomap: add caller-provided callbacks for read and readahead

Add caller-provided callbacks for read and readahead so that it can be
used generically, especially by filesystems that are not block-based.

In particular, this:
* Modifies the read and readahead interface to take in a
  struct iomap_read_folio_ctx that is publicly defined as:

  struct iomap_read_folio_ctx {
	const struct iomap_read_ops *ops;
	struct folio *cur_folio;
	struct readahead_control *rac;
	void *read_ctx;
  };

  where struct iomap_read_ops is defined as:

  struct iomap_read_ops {
      int (*read_folio_range)(const struct iomap_iter *iter,
                             struct iomap_read_folio_ctx *ctx,
                             size_t len);
      void (*read_submit)(struct iomap_read_folio_ctx *ctx);
  };

  read_folio_range() reads in the folio range and is required by the
  caller to provide. read_submit() is optional and is used for
  submitting any pending read requests.

* Modifies existing filesystems that use iomap for read and readahead to
  use the new API, through the new statically inlined helpers
  iomap_bio_read_folio() and iomap_bio_readahead(). There is no change
  in functionality for those filesystems.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>