This simplifies things quite a bit, and is reusable wherever we want to
use statx() later on. Not sure why I didn't do it like this right from
the beginning...
This allows us to properly detect mount points, for free. (Also, allows
us to respect btimes that are newer than the cutoff, which should be
useful when people untar file trees in /var/tmp)
Fixes: #16848
The kernel finally has a proper API to determine the mnt_id of a file.
Let's use it.
This adds support for the STATX_MNT_ID field of statx(), added in
kernel 5.8.