Subject: bug#59425: only dir which realpath is in /tmp is mountalbe

From: bbb ee <[email-masked]>
Date: Mon, 21 Nov 2022 20:50:58 +0100

If I access a unmountable dir from /tmp using symlink, I still can't monnt
it:
```
dev_1@dev_1 /tmp$ ln -s ~ .
dev_1@dev_1 /tmp$ cd dev_1
dev_1@dev_1 /tmp/dev_1$ df .
Filesystem     1K-blocks      Used Available Use% Mounted on
/dev/nvme0n1p7 209921236 180796556  19661028  91% /
dev_1@dev_1 /tmp/dev_1$ guix shell --container coreutils -- echo Elmo
guix shell: error: mount: mount "/home/dev_1" on
"/tmp/guix-directory.X9jW4K//home/dev_1": Invalid argument
dev_1@dev_1 /tmp/dev_1$ realpath /tmp/dev_1
/home/dev_1
```
So not all subdir of /tmp is mountable, only dir which realpath is in /tmp
is mountalbe