PCI/P2PDMA: Fix p2pmem_alloc_mmap() warning condition

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

PCI/P2PDMA: Fix p2pmem_alloc_mmap() warning condition

Commit b7e282378773 has already changed the initial page refcount of

p2pdma page from one to zero, however, in p2pmem_alloc_mmap() it uses

"VM_WARN_ON_ONCE_PAGE(!page_ref_count(page))" to assert the initial page

refcount should not be zero and the following will be reported when

CONFIG_DEBUG_VM is enabled:

page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x380400000

flags: 0x20000000002000(reserved|node=0|zone=4)

raw: 0020000000002000 ff1100015e3ab440 0000000000000000 0000000000000000

raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000

page dumped because: VM_WARN_ON_ONCE_PAGE(!page_ref_count(page))

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

WARNING: CPU: 5 PID: 449 at drivers/pci/p2pdma.c:240 p2pmem_alloc_mmap+0x83a/0xa60

Fix by using "page_ref_count(page)" as the assertion condition.