vxge: fix return of a free'd memblock on a failed dma mapping

commit 0a2c34f1 upstream.

Currently if a pci dma mapping failure is detected a free'd
memblock address is returned rather than a NULL (that indicates
an error). Fix this by ensuring NULL is returned on this error case.

Addresses-Coverity: ("Use after free")
Fixes: 528f7272

 ("vxge: code cleanup and reorganization")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>