xfs: fix pagecache truncation prior to reflink

commit 4918ef4ea008cd2ff47eb852894e3f9b9047f4f3 upstream.

Prior to remapping blocks, it is necessary to remove pages from the
destination file's page cache.  Unfortunately, the truncation is not
aggressive enough -- if page size > block size, we'll end up zeroing
subpage blocks instead of removing them.  So, round the start offset
down and the end offset up to page boundaries.  We already wrote all
the dirty data so the larger range shouldn't be a problem.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>