erofs: protect s_inodes with s_inode_list_lock for fscache

s_inodes is superblock-specific resource, which should be
protected by sb's specific lock s_inode_list_lock.

Link: https://lore.kernel.org/r/TYCP286MB23238380DE3B74874E8D78ABCA299@TYCP286MB2323.JPNP286.PROD.OUTLOOK.COM


Fixes: 7d419637 ("erofs: Support sharing cookies in the same domain")
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Jia Zhu <zhujia.zj@bytedance.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Dawei Li <set_pte_at@outlook.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>