udmabuf: pre-fault when first page fault

udmabuf: pre-fault when first page fault

The current udmabuf mmap only fills the physical memory to the
corresponding virtual address when the user actually accesses the
virtual address.

However, the current udmabuf has already obtained and pinned the folio
upon completion of the creation.This means that the physical memory has
already been acquired, rather than being accessed dynamically.

As a result, the page fault has lost its purpose as a demanding
page. Due to the fact that page fault requires trapping into kernel mode
and filling in when accessing the corresponding virtual address in mmap,
when creating a large size udmabuf, this represents a considerable
overhead.

This patch fill the pfn into page table, and then pre-fault each pfn
into vma, when first access.

Notice, if anything wrong , we do not return an error during this
pre-fault step. However, an error will be returned if the failure occurs
when the addr is truly accessed

Suggested-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Huan Yang <link@vivo.com>
Acked-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240918025238.2957823-2-link@vivo.com