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>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>