vfio/pci: Remove interrupt context counter

vfio/pci: Remove interrupt context counter

struct vfio_pci_core_device::num_ctx counts how many interrupt
contexts have been allocated. When all interrupt contexts are
allocated simultaneously num_ctx provides the upper bound of all
vectors that can be used as indices into the interrupt context
array.

With the upcoming support for dynamic MSI-X the number of
interrupt contexts does not necessarily span the range of allocated
interrupts. Consequently, num_ctx is no longer a trusted upper bound
for valid indices.

Stop using num_ctx to determine if a provided vector is valid. Use
the existence of allocated interrupt.

This changes behavior on the error path when user space provides
an invalid vector range. Behavior changes from early exit without
any modifications to possible modifications to valid vectors within
the invalid range. This is acceptable considering that an invalid
range is not a valid scenario, see link to discussion.

The checks that ensure that user space provides a range of vectors
that is valid for the device are untouched.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lore.kernel.org/lkml/20230316155646.07ae266f.alex.williamson@redhat.com/
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/e27d350f02a65b8cbacd409b4321f5ce35b3186d.1683740667.git.reinette.chatre@intel.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>