fuse: fix root lookup with nonzero generation

The root inode has a fixed nodeid and generation (1, 0).

Prior to the commit 15db1683 ("fuse: fix illegal access to inode with
reused nodeid") generation number on lookup was ignored.  After this commit
lookup with the wrong generation number resulted in the inode being
unhashed.  This is correct for non-root inodes, but replacing the root
inode is wrong and results in weird behavior.

Fix by reverting to the old behavior if ignoring the generation for the
root inode, but issuing a warning in dmesg.
Reported-by: Antonio SJ Musumeci <trapexit@spawn.link>
Closes: https://lore.kernel.org/all/CAOQ4uxhek5ytdN8Yz2tNEOg5ea4NkBb4nk0FGPjPk_9nz-VG3g@mail.gmail.com/
Fixes: 15db1683 ("fuse: fix illegal access to inode with reused nodeid")
Cc: <stable@vger.kernel.org> # v5.14
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/dir.c

@@ -391,6 +391,10 @@ int fuse_lookup_name(struct super_block *sb, u64 nodeid, const struct qstr *name
 	err = -EIO;
 	if (fuse_invalid_attr(&outarg->attr))
 		goto out_put_forget;
+	if (outarg->nodeid == FUSE_ROOT_ID && outarg->generation != 0) {
+		pr_warn_once("root generation should be zero\n");
+		outarg->generation = 0;
+	}
 
 	*inode = fuse_iget(sb, outarg->nodeid, outarg->generation,
 			   &outarg->attr, ATTR_TIMEOUT(outarg),