xen_disk: mark ioreq as mapped before unmapping in error case

Commit 4472beae modified the semantics of ioreq_{un,}map so that they are
idempotent if called when they're not needed (ie., twice in a row). However,
it neglected to handle the case where batch mapping is not being used (the
default), and one of the grants fails to map. In this case, ioreq_unmap will
be called to unwind and unmap any mappings already performed, but ioreq_unmap
simply returns due to the aforementioned change (the ioreq has not already
been marked as mapped).

The frontend user can therefore force xen_disk to leak grant mappings, a
per-domain limited resource.

Fix by marking the ioreq as mapped before calling ioreq_unmap in this
situation.

Signed-off-by: Matthew Daley <mattjd@gmail.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
