vfio: Change vfio_group->group_rwsem to a mutex

These days not much is using the read side:
 - device first open
 - ioctl_get_status
 - device FD release
 - check enforced_coherent

None of this is performance, so just make it into a normal mutex.

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/2-v1-917e3647f123+b1a-vfio_group_users_jgg@nvidia.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>