When we open a directory, we try to readahead block 0 of the directory
on the assumption that we're going to need it soon.  If the bmbt is
corrupt, the directory will never be usable and the readahead fails
immediately, so we might as well prevent the directory from being opened
at all.  This prevents a subsequent read or modify operation from
hitting it and taking the fs offline.

NOTE: We're only checking for early failures in the block mapping, not
the readahead directory block itself.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>