vfio/type1: Reduce repetitive calls in vfio_pin_pages_remote()

vfio_pin_pages_remote() is typically called to iterate over a range
of memory.  Testing CAP_IPC_LOCK is relatively expensive, so it makes
sense to push it up to the caller, which can then repeatedly call
vfio_pin_pages_remote() using that value.  This can show nearly a 20%
improvement on the worst case path through VFIO_IOMMU_MAP_DMA with
contiguous page mapping disabled.  Testing RLIMIT_MEMLOCK is much more
lightweight, but we bring it along on the same principle and it does
seem to show a marginal improvement.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Kirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>