powerpc/pseries/vas: Add VAS IRQ primary handler

irq_default_primary_handler() can be used only with IRQF_ONESHOT
flag, but the flag disables IRQ before executing the thread handler
and enables it after the interrupt is handled. But this IRQ disable
sets the VAS IRQ OFF state in the hypervisor. In case if NX faults
during this window, the hypervisor will not deliver the fault
interrupt to the partition and the user space may wait continuously
for the CSB update. So use VAS specific IRQ handler instead of
calling the default primary handler.

Increment pending_faults counter in IRQ handler and the bottom
thread handler will process all faults based on this counter.
In case if the another interrupt is received while the thread is
running, it will be processed using this counter. The synchronization
of top and bottom handlers will be done with IRQTF_RUNTHREAD flag
and will re-enter to bottom half if this flag is set.

Signed-off-by: Haren Myneni <haren@linux.ibm.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/aaad8813b4762a6753cfcd0b605a7574a5192ec7.camel@linux.ibm.com