dir_index: error out instead of BUG on corrupt dx dirs

Convert asserts (BUGs) in dx_probe from bad on-disk data to recoverable
errors with helpful warnings.  With help catching other asserts from Duane
Griffin <duaneg@dghda.com>

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Acked-by: Duane Griffin <duaneg@dghda.com>
Acked-by: Theodore Ts'o <tytso@mit.edu>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>