xen-blkback: hold write vbd-lock while swapping the vbd

xen-blkback: hold write vbd-lock while swapping the vbd

All paths holding a vbd handle or dereferencing it hold the read vbd-lock.
Swapping the device takes a write-trylock on the vbd. So, we fail a swap
if, for instance, there is an active IO operation.

Orabug: 28651655

Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Bhavesh Davda <bhavesh.davda@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>