[XFS] Fix broken HAVE_SPLICE removal commit.

Commit e687330b was meant to remove the
unused HAVE_SPLICE macro, instead an unrelated change was checked enabling
QUOTADEBUG when building DEBUG XFS. Restore the intended changes.

SGI-PV: 971046
SGI-Modid: xfs-linux-melb:xfs-kern:30924a

Signed-off-by: Donald Douwsma <donaldd@sgi.com>
Signed-off-by: Barry Naujok <bnaujok@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>