sys/gnu/vfs/ext2fs: Cleanup __KERNEL__ related macros

sys/gnu/vfs/ext2fs: Cleanup __KERNEL__ related macros

- Add comments on EXT2_XXX(s) macros. These macros on dfly ext2 happen
  to be tricky and confusing as their arg types are not consistent.
  These macros are taken from e2fsprogs header (rather than macros of
  the same name in Linux kernel header) and made to work as a part of
  kernel filesystem code.

- Remove unused __KERNEL__ version of EXT2_XXX(s) macros. These are not
  compiled on dfly (and most of them result compile error if enabled on
  dfly ext2) since __KERNEL__ is designed to be Linux kernel specific
  code while dfly ext2 is made to work with userspace version of macros
  taken from e2fsprogs as mentioned above. Moreover, some of these
  __KERNEL__ version of macros are already deprecated on Linux kernel
  and have been removed from the e2fsprogs header accordingly, so there
  is no reason to keep it here as well.

- Compiling e2fsprogs does not require ext2_fs.h installed under
  /usr/include/... as it has its own copy of the ext2_fs.h in its source
  code that the dfly ext2 originally referred to.

- This commit makes no binary difference. Also this commit does not
  affect external userspace programs.

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