vfio: add lock for hot-unplug failure handler

vfio: add lock for hot-unplug failure handler

When the sigbus handler be enabled for hot-unplug, whatever hot-unplug
sigbus or origin sigbus occur, the sigbus handler will be invoked and
it will access the bus and device. While in the control path, the vfio
req handler also will process the bus and device, so a protection of
the resources in vfio req handler should be need. This patch add a lock
in vfio req handler when process bus and device resource, to avoid the
synchronization issue when device be hot-unplugged.

Fixes: c115fd000c32 ("vfio: handle hotplug request notifier")

Signed-off-by: Jeff Guo <jia.guo@intel.com>