x86, UV: Initialize BAU hub map

Fix uninitialized uvhub_mask:

- An unitialized bit map variable was causing initialization of
  non-existant hubs (this one causes boot panics).

- And the bit map was too small for large machines.  This patch
  makes it dynamic in size.

- Fix the case where socket 0 has no enabled cpu's. Don't assume
  every hub has a socket 0.

- uv_init_per_cpu() should be __init.

Signed-off-by: Cliff Wickman <cpw@sgi.com>
Cc: <stable@kernel.org> # for .35.x
LKML-Reference: <E1Oeuyt-0004XS-0y@eag09.americas.sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>