iomap: micro optimize the ki_pos assignment in iomap_file_buffered_write

We have the new value for ki_pos right at hand in iter.pos, so assign
that instead of recalculating it from ret.

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