[PATCH] powerpc: Fix bug in bug fix for bug in lmb_alloc()

My patch (d7a5b2ff) to always panic if
lmb_alloc() fails is broken because it checks alloc < 0, but should be
checking alloc == 0.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>