cxl: Plug irq_bitmap getting leaked in cxl_context

This patch plugs the leak of irq_bitmap, allocated as part of
initialization of cxl_context struct; during the call to
afu_allocate_irqs. The bitmap is now release during the call to function
afu_release_irqs.

Reported-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
Signed-off-by: Vaibhav Jain <vaibhav@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>