xfs: remove incorrect assert in xfs_vm_writepage

xfs: remove incorrect assert in xfs_vm_writepage

In commit d5b6a504ca7e5503780e0903738a36eea307143f, titled
"xfs: simplify xfs_vm_writepage" I added an assert that any !mapped and
uptodate buffers are not dirty.  That asserts turns out to trigger a lot
when running fsx on filesystems with small block sizes.  The reason for
that is that the assert is simply incorrect.  !mapped and uptodate
just mean this buffer covers a hole, and whenever we do a set_page_dirty
we mark all blocks in the page dirty, no matter if they have data or
not.  So remove the assert, and update the comment above the condition
to match reality.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>