xen/p2m: Check __brk_limit before allocating.

xen/p2m: Check __brk_limit before allocating.

The P2M code is smart enough to return false (which means that it
cannot allocate anymore) and the error can perculate up the calling
stack without trouble - with the error logic doing the proper thing.

So check the __brk_limit values before allocating from extend_brk.

This allows us to boot on machines where we do not have enough
__brk space, and we would get this:

(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.

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

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.

With this patch, we would get now a limited amount of pages populated back:
Freeing 9f-100 pfn range: 97 pages freed
Freeing b7ee0-ecd9b pfn range: 216763 pages freed
Released 216860 pages of unused memory
Set 295297 page(s) to 1-1 mapping
Populating 100000-134f1c pfn range: 30720 pages added

[while it was instructed to populate 216860 pages back
on this particular machine]

[upstream git commit 6fc0f0142ecf25e3a7e1db52033586107f829af0]

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>