KVM: selftests: fix MAPC RDbase target formatting in vgic_lpi_stress

KVM: selftests: fix MAPC RDbase target formatting in vgic_lpi_stress

Since GITS_TYPER.PTA == 0, the ITS MAPC command demands a CPU ID,
rather than a physical redistributor address, for its RDbase
command argument.

As such, when MAPC-ing guest ITS collections, vgic_lpi_stress iterates
over CPU IDs in the range [0, nr_cpus), passing them as the RDbase
vcpu_id argument to its_send_mapc_cmd().

However, its_encode_target() in the its_send_mapc_cmd() selftest
handler expects RDbase arguments to be formatted with a 16 bit
offset, as shown by the 16-bit target_addr right shift its implementation:

        its_mask_encode(&cmd->raw_cmd[2], target_addr >> 16, 51, 16)

At the moment, all CPU IDs passed into its_send_mapc_cmd() have no
offset, therefore becoming 0x0 after the bit shift. Thus, when
vgic_its_cmd_handle_mapc() receives the ITS command in vgic-its.c,
it always interprets the RDbase target CPU as CPU 0. All interrupts
sent to collections will be processed by vCPU 0, which defeats the
purpose of this multi-vCPU test.

Fix by creating procnum_to_rdbase() helper function, which left-shifts
the vCPU parameter received by its_send_mapc_cmd 16 bits before passing
it to its_encode_target for encoding.

Signed-off-by: Maximilian Dittgen <mdittgen@amazon.de>
Link: https://patch.msgid.link/20251020145946.48288-1-mdittgen@amazon.de
Signed-off-by: Marc Zyngier <maz@kernel.org>