xfs_scrub: allow auxiliary pathnames for sandboxing

xfs_scrub: allow auxiliary pathnames for sandboxing

In the next patch, we'll tighten up the security on the xfs_scrub
service so that it can't escape.  However, sandboxing the service
involves making the host filesystem as inaccessible as possible, with
the filesystem to scrub bind mounted onto a known location within the
sandbox.  Hence we need one path for reporting and a new -M argument to
tell scrub what it should actually be trying to open.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>