xen/gntdev: convert priv->lock to a mutex

Unmapping may require sleeping and we unmap while holding priv->lock, so
convert it to a mutex.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

diff --cc drivers/xen/gntdev.c
Simple merge