xfs: fix up asserts in xfs_iflush_fork

Remove asserts in xfs_iflush_fork that would call xfs_iext_get_ext
with a potentially invalid extent buffer index.

Based on an earlier patch from Lachlan McIlroy.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Lachlan McIlroy <lmcilroy@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>