iomap: lift zeroed mapping handling into iomap_zero_range()

In preparation for special handling of subranges, lift the zeroed
mapping logic from the iterator into the caller. Since this puts the
pagecache dirty check and flushing in the same place, streamline the
comments a bit as well.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://lore.kernel.org/r/20241115200155.593665-3-bfoster@redhat.com


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