GSD-2021-1001764 mcb: fix error handling in mcb_alloc_bus()

mcb: fix error handling in mcballocbus This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v5.14.9 by commit 9fc198f415dee070a1de957bb5bf5921d8df349...