IB/hfi1: Fix the PIO_CRED credit-return mmap

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

IB/hfi1: Fix the PIO_CRED credit-return mmap

hfi1_file_mmap()'s PIO_CRED case must hand user space the single
credit-return page that holds this context's entry. That page is the
second or third page of the per-node credit-return allocation once the
hardware send context index reaches 64 or 128, so the failure below is
intermittent: when the entry lands on the first page the offset is zero
and everything works.

Two things are wrong.

First, cr_page_offset is a byte offset but .va is a struct
credit_return *, so adding it is pointer arithmetic and scales the offset
by sizeof(struct credit_return) == 64. memvirt then lands 256 KiB or
512 KiB past a 10240-byte allocation. With an IOMMU translating, that
address is inside the vmalloc range but in no vm_area, so
dma_mmap_coherent() -> iommu_dma_mmap() finds no pages, vmalloc_to_pfn()
returns page_to_pfn(NULL), and remap_pfn_range() installs a frame above
MAXPHYADDR. The first user read then takes:

psm2_ep_open_pr: Corrupted page table at address 7a14d007e000
PGD 800000013886a067 P4D 800000013886a067 PUD 13886b067 PMD 13886c067
PTE 800049168e911235
Oops: Bad pagetable: 000d [#1] SMP PTI

Second, and still wrong once the arithmetic is corrected,
dma_mmap_coherent() describes a whole coherent buffer and selects the
page within it with vma->vm_pgoff. Offsetting cpu_addr has no effect:
for a vmap'd allocation iommu_dma_mmap() uses cpu_addr only to locate the
vm_area and then maps pages[vm_pgoff], which hfi1_file_mmap() has just
set to 0. User space therefore always receives the first credit-return
page, every credit read is for the wrong context, and send PIO stalls
forever.

Use the DMA API as intended: pass the base of the allocation with its
full length and select the page with vm_pgoff. A separate length is
needed because memlen must keep describing the VMA for the existing size
check. The dma-direct path stays correct as well, since dma_direct_mmap()
adds the same vm_pgoff to the base pfn.

Tested on a Dell T7610 (Xeon E5-2650 v2, Intel IOMMU in DMA-FQ mode)
against a Threadripper PRO 3995WX peer, both Omni-Path 100. Before this
change psm2_ep_open() Oopses the kernel; with only the arithmetic
corrected psm2_ep_open() succeeds but any transfer that uses send PIO
hangs, PSM2_SDMA=2 (send PIO disabled) completing normally while
PSM2_SDMA=0 (send PIO only) hangs every time. With this change send PIO,
send DMA and the default mixed mode all work.