powerpc/vdso: Mark the vDSO code read-only after init

VDSO text is fixed-up during init so it can't be const,
but it can be read-only after init.

Do the same as x86 in commit 018ef8d ("x86/vdso: Mark the vDSO
code read-only after init") and arm in commit 11bf9b8 ("ARM/vdso:
Mark the vDSO code read-only after init"), move it into
ro_after_init section.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/e9892d288b646cbdfeef0b2b73edbaf6d3c6cabe.1734174500.git.christophe.leroy@csgroup.eu

Author: chleroy

arch/powerpc/kernel/vdso32_wrapper.S

@@ -2,7 +2,7 @@
 #include <linux/linkage.h>
 #include <asm/page.h>
 
-	__PAGE_ALIGNED_DATA
+	.section ".data..ro_after_init", "aw"
 
 	.globl vdso32_start, vdso32_end
 	.balign PAGE_SIZE

arch/powerpc/kernel/vdso64_wrapper.S

@@ -2,7 +2,7 @@
 #include <linux/linkage.h>
 #include <asm/page.h>
 
-	__PAGE_ALIGNED_DATA
+	.section ".data..ro_after_init", "aw"
 
 	.globl vdso64_start, vdso64_end
 	.balign PAGE_SIZE