xfs: xfs_seek_hole() refinement with hole searching from page cache for unwritten...

xfs_seek_hole() refinement with hole searching from page cache for unwritten extent.

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>