platform/x86: dell_rbu: fix lock imbalance in img_update_realloc

We need to ensure rbu_data.lock is always held on return.

Fixes: 289790a3ea94 ("platform/x86: dell_rbu: stop abusing the DMA API")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Stuart Hayes <stuart.w.hayes@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>