iomap: Add processed for iomap_iter

iomap: Add processed for iomap_iter

processed: The number of bytes processed by the body in the
most recent  iteration, or a negative errno. 0 causes the iteration to
stop.

The processed is useful to check when the loop breaks.

Signed-off-by: Kassey Li <quic_yingangl@quicinc.com>
Link: https://lore.kernel.org/r/20240219021138.3481763-1-quic_yingangl@quicinc.com
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>