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

commit 1401c00e59ea021c575f74612fe2dbba36d6a4ee upstream.

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>
[bwh: Backported to 3.2:
 - Adjust context
 - Drop changes to functions we don't have]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

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