arm64/mm: fix incorrect file_map_count for invalid pmd

In the Linux kernel, the following vulnerability has been resolved:

arm64/mm: fix incorrect file_map_count for invalid pmd

The page table check trigger BUG_ON() unexpectedly when split hugepage:

------------[ cut here ]------------

kernel BUG at mm/page_table_check.c:119!

Internal error: Oops - BUG: 00000000f2000800 [#1] SMP

Dumping ftrace buffer:

(ftrace buffer empty)

Modules linked in:

CPU: 7 PID: 210 Comm: transhuge-stres Not tainted 6.1.0-rc3+ #748

Hardware name: linux,dummy-virt (DT)

pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)

pc : page_table_check_set.isra.0+0x398/0x468

lr : page_table_check_set.isra.0+0x1c0/0x468

[...]

Call trace:

page_table_check_set.isra.0+0x398/0x468

__page_table_check_pte_set+0x160/0x1c0

__split_huge_pmd_locked+0x900/0x1648

__split_huge_pmd+0x28c/0x3b8

unmap_page_range+0x428/0x858

unmap_single_vma+0xf4/0x1c8

zap_page_range+0x2b0/0x410

madvise_vma_behavior+0xc44/0xe78

do_madvise+0x280/0x698

__arm64_sys_madvise+0x90/0xe8

invoke_syscall.constprop.0+0xdc/0x1d8

do_el0_svc+0xf4/0x3f8

el0_svc+0x58/0x120

el0t_64_sync_handler+0xb8/0xc0

el0t_64_sync+0x19c/0x1a0

[...]

On arm64, pmd_leaf() will return true even if the pmd is invalid due to

pmd_present_invalid() check. So in pmdp_invalidate() the file_map_count

will not only decrease once but also increase once. Then in set_pte_at(),

the file_map_count increase again, and so trigger BUG_ON() unexpectedly.

Add !pmd_present_invalid() check in pmd_user_accessible_page() to fix the

problem.