xfs: don't clear imap_valid for a non-uptodate buffers

Finding a buffer that isn't uptodate doesn't invalidate the mapping for
any given block.  The last_sector check will already take care of starting
another ioend as soon as we find any non-update buffer, and if the current
mapping doesn't include the next uptodate buffer the xfs_imap_valid check
will take care of it.

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