xen/arm32: mm: Rework is_xen_heap_page to avoid nameclash

The arm32 version of the function is_xen_heap_page currently define a
variable _mfn. This will lead to a compiler when use typesafe MFN in a
follow-up patch:

called object '_mfn' is not a function or function pointer

Fix it by renaming the local variable _mfn to mfn_.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/include/asm-arm/mm.h

@@ -140,9 +140,9 @@ extern vaddr_t xenheap_virt_start;
 #ifdef CONFIG_ARM_32
 #define is_xen_heap_page(page) is_xen_heap_mfn(page_to_mfn(page))
 #define is_xen_heap_mfn(mfn) ({                                 \
-    unsigned long _mfn = (mfn);                                 \
-    (_mfn >= mfn_x(xenheap_mfn_start) &&                        \
-     _mfn < mfn_x(xenheap_mfn_end));                            \
+    unsigned long mfn_ = (mfn);                                 \
+    (mfn_ >= mfn_x(xenheap_mfn_start) &&                        \
+     mfn_ < mfn_x(xenheap_mfn_end));                            \
 })
 #else
 #define is_xen_heap_page(page) ((page)->count_info & PGC_xen_heap)