[PATCH] Arm: tidy page_get_owner_and_nr_reference()

[Jan Beulich]

When the bumping by <nr> (instead of just 1) was introduced, a comment
was left untouched, and a bogus ASSERT_UNREACHABLE() was inserted. That
code path can in principle be taken (depending on configuration coming
from the outside), and we shouldn't assert anything we didn't check
elsewhere.

Signed-off-by: Jan Beulich <jbeul...@suse.com>
---
Noticed while reviewing the RISC-V code copying this machinery almost
verbatim.

--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -256,16 +256,13 @@ static struct domain *page_get_owner_and
 
     /* Restrict nr to avoid "double" overflow */
     if ( nr >= PGC_count_mask )
-    {
-        ASSERT_UNREACHABLE();
         return NULL;
-    }
 
     do {
         x = y;
         /*
-         * Count ==  0: Page is not allocated, so we cannot take a reference.
-         * Count == -1: Reference count would wrap, which is invalid.
+         * Count ==   0: Page is not allocated, so we cannot take a reference.
+         * Count >= -nr: Reference count would wrap, which is invalid.
          */
         if ( unlikely(((x + nr) & PGC_count_mask) <= nr) )
             return NULL;