um: hostfs: avoid issues on inode number reuse by host

Some file systems (e.g. ext4) may reuse inode numbers once the inode is
not in use anymore. Usually hostfs will keep an FD open for each inode,
but this is not always the case. In the case of sockets, this cannot
even be done properly.

As such, the following sequence of events was possible:
 * application creates and deletes a socket
 * hostfs creates/deletes the socket on the host
 * inode is still in the hostfs cache
 * hostfs creates a new file
 * ext4 on the outside reuses the inode number
 * hostfs finds the socket inode for the newly created file
 * application receives -ENXIO when opening the file

As mentioned, this can only happen if the deleted file is a special file
that is never opened on the host (i.e. no .open fop).

As such, to prevent issues, it is sufficient to check that the inode
has the expected type. That said, also add a check for the inode birth
time, just to be on the safe side.

Fixes: 74ce793b ("hostfs: Fix ephemeral inodes")
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Reviewed-by: Mickaël Salaün <mic@digikod.net>
Tested-by: Mickaël Salaün <mic@digikod.net>
Link: https://patch.msgid.link/20250214092822.1241575-1-benjamin@sipsolutions.net


Signed-off-by: Johannes Berg <johannes.berg@intel.com>