xen/balloon: Don't continue ballooning when BP_ECANCELED is encountered

Commit 3dcf6367 ("xen/balloon: cancel ballooning if adding new
memory failed") makes reserve_additional_memory() return BP_ECANCELED
when an error is encountered. This error, however, is ignored by the
caller (balloon_process()) since it is overwritten by subsequent call
to update_schedule(). This results in continuous attempts to add more
memory, all of which are likely to fail again.

We should stop trying to schedule next iteration of ballooning when
the current one has failed.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>