drm/amdkfd: Improve kfd_process lookup in kfd_ioctl

Use filep->private_data to store a pointer to the kfd_process data
structure. Take an extra reference for that, which gets released in
the kfd_release callback. Check that the process calling kfd_ioctl
is the same that opened the file descriptor. Return -EBADF if it's
not, so that this error can be distinguished in user mode.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Philip Yang <Philip.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>