xen-swiotlb: Fix wrong panic.

Propagate the baremetal git commit "swiotlb: fix wrong panic"
(fba99fa3) in the Xen-SWIOTLB version.
wherein swiotlb's map_page wrongly calls panic() when it can't find
a buffer fit for device's dma mask.  It should return an error instead.

Devices with an odd dma mask (i.e.  under 4G) like b44 network card hit
this bug (the system crashes):

http://marc.info/?l=linux-kernel&m=129648943830106&w=2



If xen-swiotlb returns an error, b44 driver can use the own bouncing
mechanism.

CC: stable@kernel.org
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>