xfs: test xfs_db whether still can get corrupted field value

xfs: test xfs_db whether still can get corrupted field value

This is a regression test that xfs_db code doesn't clean up the
LIBXFS_READBUF_SALVAGE call sites in set_cur so that we can't use the
return value directly and scraping it out later.

Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>