misc: genwqe: Fix potential memory leak when pinning memory

Fix a memory leak in the genwqe_pin_mem() error path as called by
ioctl GENWQE_PIN_MEM. In case there is an error encountered when
mapping memory, the already allocated dma_mapping struct needs to
be freed correctly.

Detected by Coverity: CID 1162606.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Acked-by: Frank Haverkamp <haver@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>