mm/memory_hotplug: Enable kernel page-tables before free pages

When pages are getting free'ed, the kernel assumes that the page-table
entries were enabled, since the pages were in-use before. And along the
free path, with PAGE_POISONING enabled, it poisons the pages and then
disables the entries. However, this is not the case with memory hotplug.
Initially, when the hotplugable memory is added, the page-table entries
are created and enabled, and when onlined, the pages get free'ed and the
entries get disabled. These pages now remain disabled along the path of
doing a offline and online again and we hit data abort since CPU is trying
to poison the pages whose entries are disabled.

	[243.736338] Unable to handle kernel paging request at virtual address
	 ffffffdde0000000
	[243.744577] Mem abort info:
	[243.747491]   Exception class = DABT (current EL), IL = 32 bits
	[243.753638]   SET = 0, FnV = 0
	[243.756818]   EA = 0, S1PTW = 0
	[243.760251] Data abort info:
	[243.763473]   ISV = 0, ISS = 0x00000047
	[243.767446]   CM = 0, WnR = 1
	[243.770584] swapper pgtable: 4k pages, 39-bit VAs, pgd = ffffff97159d7000
	[243.777586] [ffffffdde0000000] *pgd=00000001afc95003, *pud=00000001afc95003,
	 *pmd=00000001aeffe003, *pte=00f80001e0000712
	[243.788985] Internal error: Oops: 96000047 [#1] PREEMPT SMP
	[243.794717] Modules linked in:
	[243.797873] CPU: 2 PID: 4538 Comm: sh Tainted: G S 4.14.62+ #862
	...
	[  369.546983] Call trace:
	[  369.549508]  __memset+0x108/0x200
	[  369.552929]  __free_pages_ok+0x1ac/0x480
	[  369.556970]  __free_pages+0x48/0x64
	[  369.560560]  online_pages_range+0x124/0x16c
	[  369.564863]  walk_system_ram_range+0x98/0xe4
	[  369.569257]  online_pages+0xc4/0x1fc
	[  369.572948]  memory_block_change_state+0x118/0x18c
	[  369.577879]  memory_subsys_online+0x48/0x5c
	[  369.582189]  device_online+0x74/0x98
	[  369.585869]  store_mem_state+0xa4/0xf4
	[  369.589730]  dev_attr_store+0x40/0x58
	[  369.593508]  sysfs_kf_write+0x50/0x68
	[  369.597282]  kernfs_fop_write+0x12c/0x1cc
	[  369.601408]  __vfs_write+0x54/0x14c

Hence, enable the kernel page-tables before free'ing the pages.

Change-Id: I57838e296c8836def5a73d43329683ce9dbf1cd7
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>