commit 1401c00e 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>
Cc: Ian Campbell <ian.campbell@citrix.com>
[ luis: 3.16 prereq for:
  30b03d05 xen/gntdevt: Fix race condition in gntdev_release() ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>