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.

Fixes: 7d75bbb4 ("irqchip/gic-v3-its: Add VPE irq domain allocation/teardown")
Signed-off-by: Kaige Fu <kaige.fu@linux.alibaba.com>
[maz: fixed commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/d9e36dee512e63670287ed9eff884a5d8d6d27f2.1631672311.git.kaige.fu@linux.alibaba.com