kcore: add _text to KCORE_TEXT

Extend KCORE_TEXT to cover the pages between _text and _stext, to allow examining some important page table pages.

readelf -a output on x86_64 before and after patch:
Type Offset VirtAddr PhysAddr

before    LOAD           0x00007fff8100c000 0xffffffff81009000 0x0000000000000000
after     LOAD           0x00007fff81003000 0xffffffff81000000 0x0000000000000000

The newly covered pages are:

	0xffffffff81000000 <startup_64> etc.
	0xffffffff81001000 <init_level4_pgt>
	0xffffffff81002000 <level3_ident_pgt>
	0xffffffff81003000 <level3_kernel_pgt>
	0xffffffff81004000 <level2_fixmap_pgt>
	0xffffffff81005000 <level1_fixmap_pgt>
	0xffffffff81006000 <level2_ident_pgt>
	0xffffffff81007000 <level2_kernel_pgt>
	0xffffffff81008000 <level2_spare_pgt>

Before patch, /proc/kcore shows outdated contents for the above page table pages, for example:

	(gdb) p level3_ident_pgt
	$1 = {<text variable, no debug info>} 0xffffffff81002000 <level3_ident_pgt>
	(gdb) p/x *((pud_t *)&level3_ident_pgt)@512
	$2 = {{pud = 0x1006063}, {pud = 0x0} <repeats 511 times>}

while the real content is:

	root@hp /home/wfg# hexdump -s 0x1002000 -n 4096 /dev/mem
	1002000 6063 0100 0000 0000 8067 0000 0000 0000
	1002010 0000 0000 0000 0000 0000 0000 0000 0000
	*
	1003000

That is, on a x86_64 box with 2GB memory, we can see first-1GB / full-2GB identity mapping before/after patch:

	(gdb) p/x *((pud_t *)&level3_ident_pgt)@512

before $1 = {{pud = 0x1006063}, {pud = 0x0} <repeats 511 times>}
after $1 = {{pud = 0x1006063}, {pud = 0x8067}, {pud = 0x0} <repeats 510 times>}

Obviously the content before patch is wrong.

---
 fs/proc/kcore.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -587,7 +587,7 @@ static struct kcore_list kcore_text;
  */
 static void __init proc_kcore_text_init(void)
 {
-	kclist_add(&kcore_text, _stext, _end - _stext, KCORE_TEXT);
+	kclist_add(&kcore_text, _text, _end - _text, KCORE_TEXT);
 }
 #else
 static void __init proc_kcore_text_init(void)

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