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.