arm64: assign page_offset with VA_BITS kernel configuration value

On RHEL9, crash hits a bug when executing "crash /proc/kcore":
seek error: kernel virtual address: ffff6a0f3fff0000 type: "pmd page"

The kernel virtual address does not vary with vabits_actual, instead,
is determined by configuration value. But crash does not observe this
fact.

Since vabits_actual related kernel commit is introduced after arm64
mm layout flip commit, so changes are safe under the condition if
(ms->VA_BITS_ACTUAL), and keep the else branch untouched.

Signed-off-by: Pingfan Liu <piliu@redhat.com>

arm64.c

@@ -220,9 +220,10 @@ arm64_init(int when)
 
 		/* vabits_actual introduced after mm flip, so it should be flipped layout */
 		if (ms->VA_BITS_ACTUAL) {
-			ms->page_offset = ARM64_FLIP_PAGE_OFFSET_ACTUAL;
-			machdep->identity_map_base = ARM64_FLIP_PAGE_OFFSET_ACTUAL;
-			machdep->kvbase = ARM64_FLIP_PAGE_OFFSET_ACTUAL;
+			ms->page_offset = ARM64_FLIP_PAGE_OFFSET;
+			/* useless on arm64 */
+			machdep->identity_map_base = ARM64_FLIP_PAGE_OFFSET;
+			machdep->kvbase = ARM64_FLIP_PAGE_OFFSET;
 			ms->userspace_top = ARM64_USERSPACE_TOP_ACTUAL;
 		} else {
 			ms->page_offset = ARM64_PAGE_OFFSET;

defs.h

@@ -3234,6 +3234,7 @@ typedef signed int s32;
 #define ARM64_PAGE_OFFSET    ((0xffffffffffffffffUL) \
 					<< (machdep->machspec->VA_BITS - 1))
 /* kernels >= v5.4 the kernel VA space is flipped */
+#define ARM64_FLIP_PAGE_OFFSET (-(1UL) << machdep->machspec->CONFIG_ARM64_VA_BITS)
 #define ARM64_FLIP_PAGE_OFFSET_ACTUAL ((0xffffffffffffffffUL) \
 					- ((1UL) << machdep->machspec->VA_BITS_ACTUAL) + 1)