erofs: fix .fadvise() for page cache sharing

Currently, .fadvise() doesn't work well if page cache sharing is on
since shared inodes belong to a pseudo fs generated with init_pseudo(),
and sb->s_bdi is the default one &noop_backing_dev_info.

Then, generic_fadvise() will just behave as a no-op if sb->s_bdi is
&noop_backing_dev_info, but as the bdev fs (the bdev fs changes
inode_to_bdi() instead), it's actually NOT a pure memfs.

Let's generate a real bdi for erofs_ishare_mnt instead.

Fixes: d86d781 ("erofs: implement .fadvise for page cache share")
Reviewed-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Author: hsiangkao

fs/erofs/ishare.c

@@ -200,8 +200,19 @@ struct inode *erofs_real_inode(struct inode *inode, bool *need_iput)
 
 int __init erofs_init_ishare(void)
 {
-	erofs_ishare_mnt = kern_mount(&erofs_anon_fs_type);
-	return PTR_ERR_OR_ZERO(erofs_ishare_mnt);
+	struct vfsmount *mnt;
+	int ret;
+
+	mnt = kern_mount(&erofs_anon_fs_type);
+	if (IS_ERR(mnt))
+		return PTR_ERR(mnt);
+	/* generic_fadvise() doesn't work if s_bdi == &noop_backing_dev_info */
+	ret = super_setup_bdi(mnt->mnt_sb);
+	if (ret)
+		kern_unmount(mnt);
+	else
+		erofs_ishare_mnt = mnt;
+	return ret;
 }
 
 void erofs_exit_ishare(void)