Revert "x86, mm: Set NX across entire PMD at boot"

This reverts commit a5c187d9 which
was commit 45e2a9d4 upstream.

The previous commit caused suspend/resume to stop working on at least
some systems - specifically, the system would reboot when woken.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Steven Rostedt <rostedt@goodmis.org>

arch/x86/mm/init_64.c

@@ -778,7 +778,7 @@ void mark_rodata_ro(void)
 	unsigned long text_end = PAGE_ALIGN((unsigned long) &__stop___ex_table);
 	unsigned long rodata_end = PAGE_ALIGN((unsigned long) &__end_rodata);
 	unsigned long data_start = (unsigned long) &_sdata;
-	unsigned long all_end;
+	unsigned long all_end = PFN_ALIGN(&_end);
 
 	printk(KERN_INFO "Write protecting the kernel read-only data: %luk\n",
 	       (end - start) >> 10);
@@ -789,16 +789,7 @@ void mark_rodata_ro(void)
 	/*
 	 * The rodata/data/bss/brk section (but not the kernel text!)
 	 * should also be not-executable.
-	 *
-	 * We align all_end to PMD_SIZE because the existing mapping
-	 * is a full PMD. If we would align _brk_end to PAGE_SIZE we
-	 * split the PMD and the reminder between _brk_end and the end
-	 * of the PMD will remain mapped executable.
-	 *
-	 * Any PMD which was setup after the one which covers _brk_end
-	 * has been zapped already via cleanup_highmem().
 	 */
-	all_end = roundup((unsigned long)_brk_end, PMD_SIZE);
 	set_memory_nx(rodata_start, (all_end - rodata_start) >> PAGE_SHIFT);
 
 	rodata_test();