repair: fix segv on directory block read failure

repair: fix segv on directory block read failure

We try to read all blocks in the directory, but if we have a block
form directory we only have one block and so we need to fail if
there is a read error. Otherwise we try to derefence a null buffer
pointer.

While fixing the error handling for a read failure, fix the bug that
caused the read failure - trying to verify a block format buffer
with the data format buffer verifier.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>