vfio: powerpc/spapr: Move locked_vm accounting to helpers

There moves locked pages accounting to helpers.
Later they will be reused for Dynamic DMA windows (DDW).

This reworks debug messages to show the current value and the limit.

This stores the locked pages number in the container so when unlocking
the iommu table pointer won't be needed. This does not have an effect
now but it will with the multiple tables per container as then we will
allow attaching/detaching groups on fly and we may end up having
a container with no group attached but with the counter incremented.

While we are here, update the comment explaining why RLIMIT_MEMLOCK
might be required to be bigger than the guest RAM. This also prints
pid of the current process in pr_warn/pr_debug.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
[aw: for the vfio related changes]
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>