irqchip/gic-v3-its: Fix potential VPE leak on error

In the Linux kernel, the following vulnerability has been resolved:

irqchip/gic-v3-its: Fix potential VPE leak on error

In its_vpe_irq_domain_alloc, when its_vpe_init() returns an error,

there is an off-by-one in the number of VPEs to be freed.

Fix it by simply passing the number of VPEs allocated, which is the

index of the loop iterating over the VPEs.

[maz: fixed commit message]