xfs: #ifdef out perag code for userspace

The xfs_perag structure and initialization is unused in userspace,
so #ifdef it out with __KERNEL__ to facilitate the xfsprogs sync
and build.

Signed-off-by: Eric Sandeen <esandeen@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>