xfs_scrub: use the display mountpoint for reporting file corruptions

xfs_scrub: use the display mountpoint for reporting file corruptions

In systemd service mode, we make systemd bind-mount the target
mountpoint onto /tmp/scrub (/tmp is private to the service) so that
updates to the global mountpoint in the shared mount namespace don't
propagate into our service container and vice versa, and pass the path
to the bind mount to xfs_scrub via -M.  This solves races such as
unmounting of the target mount point after service container creation
but before process invocation that result in the wrong filesystem being
scanned.

IOWs, to scrub /usr, systemd runs "xfs_scrub -M /tmp/scrub /usr".
Pretend that /usr is a separate filesystem.

However, when xfs_scrub snapshots the handle of /tmp/scrub, libhandle
remembers that /tmp/scrub the beginning of the path, not the pathname
that we want to use for reporting (/usr).  This means that
handle_to_path returns /tmp/scrub and not /usr as well, with the
unfortunate result that file corrupts are reported with the pathnames in
the xfs_scrub@ service container, not the global ones.

Put another way, xfs_scrub should complain that /usr/bin/X is corrupt,
not /tmp/scrub/bin/X.

Therefore, modify scrub_render_ino_descr to manipulate the path buffer
during error reporting so that the user always gets the mountpoint
passed in, even if someone tells us to use another path for the actual
open() call in phase 1.

Cc: <linux-xfs@vger.kernel.org> # v6.10.0
Fixes: 9a8b09762f9a52 ("xfs_scrub: use parent pointers when possible to report file operations")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>