Relinquish fscache volume with mutex held.  Otherwise if a new domain is
registered when the old domain with the same name gets removed from the
list but not relinquished yet, fscache may complain the collision.

Fixes: 8b7adf1d ("erofs: introduce fscache-based domain")
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Jia Zhu <zhujia.zj@bytedance.com>
Link: https://lore.kernel.org/r/20230209063913.46341-4-jefflexu@linux.alibaba.comSigned-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>