iomap: reset per-iter state on non-error iter advances

iomap: reset per-iter state on non-error iter advances

iomap_iter_advance() zeroes the processed and mapping fields on
every non-error iteration except for the last expected iteration
(i.e. return 0 expected to terminate the iteration loop). This
appears to be circumstantial as nothing currently relies on these
fields after the final iteration.

Therefore to better faciliate iomap_iter reuse in subsequent
patches, update iomap_iter_advance() to always reset per-iteration
state on successful completion.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://lore.kernel.org/r/20241115200155.593665-2-bfoster@redhat.com
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>