PCI/P2PDMA: Reset page reference count when page mapping fails

PCI/P2PDMA: Reset page reference count when page mapping fails

When mapping a p2pdma page the page reference count is initialised to 1
prior to calling vm_insert_page(). This is to avoid vm_insert_page()
warning if the page refcount is zero. Prior to setting the page count there
is a check to ensure the page is currently free (ie. has a zero reference
count).

However vm_insert_page() can fail. In this case the pages are freed back to
the genalloc pool, but that does not reset the page refcount.  So a future
allocation of the same page will see the elevated page refcount from the
previous set_page_count() call triggering the VM_WARN_ON_ONCE_PAGE checking
that the page is free.

Fix this by resetting the page refcount to zero using set_page_count().
Note that put_page() is not used because that would result in freeing the
page twice due to implicitly calling p2pdma_folio_free().

Fixes: b7e282378773 ("mm/mm_init: move p2pdma page refcount initialisation to p2pdma")
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Acked-by: Balbir Singh <balbirs@nvidia.com>
Link: https://patch.msgid.link/20260112005440.998543-1-apopple@nvidia.com