Bugfix: Minimize risk of spurious filesystem loops with `mount`

When a backup contains a directory that has the same name as its parent, say
`a/b/b`, and the GNU `find` command was run on this backup in a restic mount,
`find` would refuse to traverse the lowest `b` directory, instead printing
`File system loop detected`. This was due to the way the restic mount command
generates inode numbers for directories in the mount point.

The rule for generating these inode numbers was changed in 0.15.0. It has
now been changed again to avoid this issue. A perfect rule does not exist,
but the probability of this behavior occurring is now extremely small.

When it does occur, the mount point is not broken, and scripts that traverse
the mount point should work as long as they don't rely on inode numbers for
detecting filesystem loops.

https://github.com/restic/restic/issues/4253
https://github.com/restic/restic/pull/4255