iomap: cleanup the pending writeback tracking in iomap_writepage_map_blocks

iomap: cleanup the pending writeback tracking in iomap_writepage_map_blocks

We don't care about the count of outstanding ioends, just if there is one.
Replace the count variable passed to iomap_writepage_map_blocks with a
boolean to make that more clear.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
[hch: rename the variable, update the commit message]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/20250710133343.399917-4-hch@lst.de
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>