drm/amdkfd: Migrate in CPU page fault use current mm

drm/amdkfd: Migrate in CPU page fault use current mm

migrate_vma_setup shows below warning because we don't hold another
process mm mmap_lock. We should use current vmf->vma->vm_mm instead, the
caller already hold current mmap lock inside CPU page fault handler.

 WARNING: CPU: 10 PID: 3054 at include/linux/mmap_lock.h:155 find_vma
 Call Trace:
  walk_page_range+0x76/0x150
  migrate_vma_setup+0x18a/0x640
  svm_migrate_vram_to_ram+0x245/0xa10 [amdgpu]
  svm_migrate_to_ram+0x36f/0x470 [amdgpu]
  do_swap_page+0xcfe/0xec0
  __handle_mm_fault+0x96b/0x15e0
  handle_mm_fault+0x13f/0x3e0
  do_user_addr_fault+0x1e7/0x690

Fixes: e1f84eef313f ("drm/amdkfd: handle CPU fault on COW mapping")
Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>