xfs: rename the offset variable in xfs_writepage_map

Calling it file_offset makes the usage more clear, especially with
a new poffset variable that will be added soon for the offset inside
the page.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>