x86/head: Make empty_zero_page read-only

This makes the zero page read-only.  It should have essentially no
overhead because it lives in the shiny new .robss..page_aligned
section.

While I'm at it, use ENDOBJECT to make the objdump output nicer.

Signed-off-by: Andy Lutomirski <luto@kernel.org>

arch/x86/kernel/head_32.S

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index a2c7f1d..d717182 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -558,12 +558,16 @@
 #endif
 initial_pg_fixmap:
 	.fill 1024,4,0
-.globl empty_zero_page
-empty_zero_page:
-	.fill 4096,1,0
 .globl swapper_pg_dir
 swapper_pg_dir:
 	.fill 1024,4,0
+
+__PAGE_ALIGNED_ROBSS
+.globl empty_zero_page
+.align PAGE_SIZE
+empty_zero_page:
+	.fill 4096,1,0
+ENDOBJECT(empty_zero_page)
 EXPORT_SYMBOL(empty_zero_page)
 
 /*

arch/x86/kernel/head_64.S

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index b467b14..f13eb18 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -496,8 +496,9 @@
 
 #include "../../x86/xen/xen-head.S"
 	
-	__PAGE_ALIGNED_BSS
+	__PAGE_ALIGNED_ROBSS
 NEXT_PAGE(empty_zero_page)
 	.skip PAGE_SIZE
+ENDOBJECT(empty_zero_page)
 EXPORT_SYMBOL(empty_zero_page)