iomap: simplify io_flags and io_type in struct iomap_ioend

The ioend fields for distinct types of I/O are a bit complicated.
Consolidate them into a single io_flag field with it's own flags
decoupled from the iomap flags.  This also prepares for adding a new
flag that is unrelated to both of the iomap namespaces.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250206064035.2323428-3-hch@lst.de


Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>