[SCSI] hpsa: fix memory leak in CCISS_BIG_PASSTHRU ioctl

We were leaking a command buffer if a DMA mapping error was
encountered in the CCISS_BIG_PASSTHRU ioctl.

Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>