libxfs: make libxfs_buf_read_map return an error code
Make libxfs_buf_read_map() and libxfs_readbuf() return an error code
instead of making callers guess what happened based on whether or not
they got a buffer back.
Add a new SALVAGE flag so that certain utilities (xfs_db and xfs_repair)
can attempt salvage operations even if the verifiers failed, which was
the behavior before this change.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Eric Sandeen <sandeen@sandeen.net>