iomap: Simplify iomap_writepage_map()

Rename end_offset to end_pos and file_offset to pos to match the rest
of the file.  Simplify the loop by calculating nblocks up front instead
of each time around the loop.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>