slub page alloc fallback: Enable interrupts for GFP_WAIT.

The fallback path needs to enable interrupts like done for
the other page allocator calls. This was not necessary with
the alternate fast path since we handled irq enable/disable in
the slow path. The regular fastpath handles irq enable/disable
around calls to the slow path so we need to restore the proper
status before calling the page allocator from the slowpath.

Signed-off-by: Christoph Lameter <clameter@sgi.com>