minix: describe usage of different magic numbers

One can get this information from minix/inode.c, but adding the
explanations at the definition sites is more appropriate.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>