xfs_db: clean up the salvage read callsites in set_cur()

xfs_db: clean up the salvage read callsites in set_cur()

Clean up the LIBXFS_READBUF_SALVAGE call sites in set_cur so that we
use the return value directly instead of scraping it out later.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>