xfs: xfs_seek_data() refinement with unwritten extents check up from page cache

xfs_seek_data() refinement with unwritten extents check up from page cache.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>