mshv: Fix missing error code on VP allocation failure

mshv: Fix missing error code on VP allocation failure

In mshv_partition_ioctl_create_vp(), when kzalloc for the VP struct
fails, the code jumps to the cleanup path without setting ret. At that
point ret is 0 from the preceding successful mshv_vp_stats_map() call,
so the function returns success to userspace despite having failed to
create the VP. No fd is installed and no VP is registered in pt_vp_array,
but userspace has no way to know the operation failed.

Set ret to -ENOMEM before jumping to the cleanup path.

Fixes: 621191d709b14 ("Drivers: hv: Introduce mshv_root module to expose /dev/mshv to VMMs")
Signed-off-by: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>
Reviewed-by: Anirudh Rayabharam (Microsoft) <anirudh@anirudhrb.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>