xen/p2m: Allow alloc_p2m_middle to call reserve_brk depending on argument

For identity cases we want to call reserve_brk only on the boundary
conditions of the middle P2M (so P2M[x][y][0] = extend_brk). This is
to work around identify regions (PCI spaces, gaps in E820) which are not
aligned on 2MB regions.

However for the case were we want to allocate P2M middle leafs at the
early bootup stage, irregardless of this alignment check we need some
means of doing that.  For that we provide the new argument.
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/p2m.c

@@ -499,7 +499,7 @@ static bool alloc_p2m(unsigned long pfn)
 	return true;
 }
 
-static bool __init early_alloc_p2m_middle(unsigned long pfn)
+static bool __init early_alloc_p2m_middle(unsigned long pfn, bool check_boundary)
 {
 	unsigned topidx, mididx, idx;
 
@@ -508,7 +508,7 @@ static bool __init early_alloc_p2m_middle(unsigned long pfn)
 	idx = p2m_index(pfn);
 
 	/* Pfff.. No boundary cross-over, lets get out. */
-	if (!idx)
+	if (!idx && check_boundary)
 		return false;
 
 	WARN(p2m_top[topidx][mididx] == p2m_identity,
@@ -592,8 +592,8 @@ unsigned long __init set_phys_range_identity(unsigned long pfn_s,
 		WARN_ON(!early_alloc_p2m(pfn));
 	}
 
-	early_alloc_p2m_middle(pfn_s);
-	early_alloc_p2m_middle(pfn_e);
+	early_alloc_p2m_middle(pfn_s, true);
+	early_alloc_p2m_middle(pfn_e, true);
 
 	for (pfn = pfn_s; pfn < pfn_e; pfn++)
 		if (!__set_phys_to_machine(pfn, IDENTITY_FRAME(pfn)))