xfs: remove unused parameter from xfs_writepage_map

The first thing that xfs_writepage_map does is clobber the offset
parameter.  Since we never use the passed-in value, turn the parameter
into a local variable.  This gets rid of an UBSAN warning in generic/466.

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