[XFS] XFS sysctl cleanups

Removes unneeded sysctl insert at head behaviour. Cleans up sysctl
definitions to use C99 initialisers. Patch provided by Eric W. Biederman.

SGI-PV: 960192
SGI-Modid: xfs-linux-melb:xfs-kern:28031a

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>