vfio/type1: Restore mapping performance with mdev support

As part of the mdev support, type1 now gets a task reference per
vfio_dma and uses that to get an mm reference for the task while
working on accounting.  That's correct, but it's not fast.  For some
paths, like vfio_pin_pages_remote(), we know we're only called from
user context, so we can restore the lighter weight calls.  In other
cases, we're effectively already testing whether we're in the stored
task context elsewhere, extend this vfio_lock_acct() as well.

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