xfs: kill meaningless variable 'zero'

In xfs_file_aio_write_checks(), variable 'zero' is there only to
satisfy xfs_zero_eof(), the result of it is ignored. Now, with
iomap_zero_range() based xfs_zero_eof(), we can safely pass NULL as
the last param of it and kill 'zero'.

Signed-off-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>