xen/p2m: Reserve 8MB of _brk space for P2M leafs when populating back.

When we release pages back during bootup:

Freeing  9d-100 pfn range: 99 pages freed
Freeing  9cf36-9d0d2 pfn range: 412 pages freed
Freeing  9f6bd-9f6bf pfn range: 2 pages freed
Freeing  9f714-9f7bf pfn range: 171 pages freed
Freeing  9f7e0-9f7ff pfn range: 31 pages freed
Freeing  9f800-100000 pfn range: 395264 pages freed
Released 395979 pages of unused memory

We then try to populate those pages back. In the P2M tree however
the space for those leafs must be reserved - as such we use extend_brk.
We reserve 8MB of _brk space, which means we can fit over
1048576 PFNs - which is more than we should ever need.

Without this, on certain compilation of the kernel we would hit:

(XEN) domain_crash_sync called from entry.S
(XEN) CPU:    0
(XEN) RIP:    e033:[<ffffffff818aad3b>]
(XEN) RFLAGS: 0000000000000206   EM: 1   CONTEXT: pv guest
(XEN) rax: ffffffff81a7c000   rbx: 000000000000003d   rcx: 0000000000001000
(XEN) rdx: ffffffff81a7b000   rsi: 0000000000001000   rdi: 0000000000001000
(XEN) rbp: ffffffff81801cd8   rsp: ffffffff81801c98   r8:  0000000000100000
(XEN) r9:  ffffffff81a7a000   r10: 0000000000000001   r11: 0000000000000003
(XEN) r12: 0000000000000004   r13: 0000000000000004   r14: 000000000000003d
(XEN) r15: 00000000000001e8   cr0: 000000008005003b   cr4: 00000000000006f0
(XEN) cr3: 0000000125803000   cr2: 0000000000000000
(XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: e02b   cs: e033
(XEN) Guest stack trace from rsp=ffffffff81801c98:

.. which is extend_brk hitting a BUG_ON.

Interestingly enough, most of the time we are not going to hit this
b/c the _brk space is quite large (v3.5):
 ffffffff81a25000 B __brk_base
 ffffffff81e43000 B __brk_limit
= ~4MB.

vs earlier kernels (with this back-ported), the space is smaller:
 ffffffff81a25000 B __brk_base
 ffffffff81a7b000 B __brk_limit
= 344 kBytes.

where we would certainly hit this and hit extend_brk.

Note that git commit c3d93f880197953f86ab90d9da4744e926b38e33
(xen: populate correct number of pages when across mem boundary (v2))
exposed this bug).

[v1: Made it 8MB of _brk space instead of 4MB per Jan's suggestion]

CC: stable@vger.kernel.org #only for 3.5
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>