Rather than using a single constant PERCPU_ENOUGH_ROOM, compute it as
the sum of kernel_percpu + PERCPU_MODULE_RESERVE. This is now common
to all architectures; if an architecture wants to set
PERCPU_ENOUGH_ROOM to something special, then it may do so (ia64 is
the only one which does).

Signed-off-by: Jeremy Fitzhardinge
Signed-off-by: Andi Kleen
Cc: Rusty Russell
Cc: Eric W. Biederman
Cc: Andi Kleen