iommufd: Take dma_resv lock before dma_buf_unpin() in release path

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

iommufd: Take dma_resv lock before dma_buf_unpin() in release path

dma_buf_unpin() requires the caller to hold the exporter's dma_resv
lock:

void dma_buf_unpin(struct dma_buf_attachment *attach)
{
...
dma_resv_assert_held(dmabuf->resv);
...
}

iopt_release_pages() calls dma_buf_unpin() without taking that lock,
so every iommufd_ioas_destroy()/iommufd_ioas_unmap() that releases
the last reference on a DMABUF-backed iopt_pages triggers a WARN.
This was hit while running tools/testing/selftests/iommu/iommufd:

WARNING: drivers/dma-buf/dma-buf.c:1137 at dma_buf_unpin+0x62/0x70
RIP: 0010:dma_buf_unpin+0x62/0x70
Call Trace:
<TASK>
dma_buf_unpin+0x62/0x70
iopt_release_pages+0xe4/0x190
iopt_unmap_iova_range+0x1c7/0x290
iopt_unmap_all+0x1a/0x30
iommufd_ioas_destroy+0x1d/0x50
iommufd_fops_release+0x93/0x150
__fput+0xfc/0x2c0
__x64_sys_close+0x3d/0x80
do_syscall_64+0x65/0x180
</TASK>

Take the dma_resv lock around dma_buf_unpin() in iopt_release_pages(),
matching the iopt_map_dmabuf() convention. dma_buf_detach() acquires the
reservation lock internally, so it must remain outside the locked region.