iomap: Refactor iomap_write_delalloc_punch() function out

This patch factors iomap_write_delalloc_punch() function out. This function
is resposible for actual punch out operation.
The reason for doing this is, to avoid deep indentation when we bring
punch-out of individual non-dirty blocks within a dirty folio in a later
patch (which adds per-block dirty status handling to iomap) to avoid
delalloc block leak.

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>