PCI/P2PDMA: Release per-CPU pgmap ref when vm_insert_page() fails

PCI/P2PDMA: Release per-CPU pgmap ref when vm_insert_page() fails

When vm_insert_page() fails in p2pmem_alloc_mmap(), p2pmem_alloc_mmap()
doesn't invoke percpu_ref_put() to free the per-CPU ref of pgmap acquired
after gen_pool_alloc_owner(), and memunmap_pages() will hang forever when
trying to remove the PCI device.

Fix it by adding the missed percpu_ref_put().

Fixes: 7e9c7ef83d78 ("PCI/P2PDMA: Allow userspace VMA allocations through sysfs")
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Link: https://patch.msgid.link/20251220040446.274991-2-houtao@huaweicloud.com