repair: handle directory block corruption in phase 6

repair: handle directory block corruption in phase 6

This should only occur in no-modify mode, but when we fail to find
the last extent in a directory btree due to corruption we need to
trash the directory if it's the first data block we find the error
on. That is because there is nothing to recover from the directory,
and if we try to scan it xfs_reapir segv's because nothing has been
read from disk.

Also catch a memory allocation failure in this code, too.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>