xfs_repair: collapse 2 cases in process_sf_dir2

xfs_repair: collapse 2 cases in process_sf_dir2

process_sf_dir2() has 2 cases for a bad namelen: on-disk namelen
is 0, or on-disk namelen extends beyond the directory i_size.

After the prior 2 patches, the code for each case is now essentially
the same, so collapse the two cases.

Note, this does slightly change some of the error messages.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>