xfs: turn NLINK feature on by default

mkfs has turned on the XFS_SB_VERSION_NLINKBIT feature bit by
default since November 2007. It's about time we simply made the
kernel code turn it on by default and so always convert v1 inodes to
v2 inodes when reading them in from disk or allocating them. This
This removes needless version checks and modification when bumping
link counts on inodes, and will take code out of a few common code
paths.

   text    data     bss     dec     hex filename
 783251  100867     616  884734   d7ffe fs/xfs/xfs.o.orig
 782664  100867     616  884147   d7db3 fs/xfs/xfs.o.patched

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>