vfio: Fix memory leak in vfio_group_create()

When snprintf() fails in vfio_group_create(), the function returns
NULL without freeing the allocated 'group' structure, causing a memory
leak.
Fix this by jumping to the existing err_free_group label.

Signed-off-by: Zongmin Zhou <zhouzongmin@kylinos.cn>
Signed-off-by: Will Deacon <will@kernel.org>