iomap: drop unnecessary pos param from iomap_write_[begin|end]

iomap_write_begin() and iomap_write_end() both take the iter and
iter->pos as parameters. Drop the unnecessary pos parameter and
sample iter->pos within each function.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://lore.kernel.org/20250506134118.911396-3-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>