sys/gnu/vfs/ext2fs: Remove|fix duplicated macro definitions

sys/gnu/vfs/ext2fs: Remove|fix duplicated macro definitions

- Define MAXNAMLEN using EXT2_NAME_LEN since EXT2_NAME_LEN
  is the original ondisk definition.

- Bring back MAXSYMLINKLEN that was removed in a0403c0
  using EXT2_MAXSYMLINKLEN. It's probably better to keep both
  EXT2_MAXSYMLINKLEN and MAXSYMLINKLEN just like above MAXNAMLEN
  does considering it's been exposed to userspace and removing
  may break them (although there are probably none or very few).

- Remove duplicated MAXMNTLEN definition.

- (Note that dfly's ext2 seems to be broken currently)