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

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>