RDMA/ucma: Fix resource leak on error path

In ucma_process_join(), if the call to xa_alloc() fails, the function will
return without freeing mc. Fix this by jumping to the correct line.

In the process I renamed the jump labels to something more memorable for
extra clarity.

Link: https://lore.kernel.org/r/20200902162454.332828-1-alex.dewar90@gmail.com
Addresses-Coverity-ID: 1496814 ("Resource leak")
Fixes: 95fe5109

 ("RDMA/ucma: Remove mc_list and rely on xarray")
Signed-off-by: Alex Dewar <alex.dewar90@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>