fs/affs: define AFFSNAMEMAX to replace constant use

30 was used all over the place to compare name length against
AFFS maximum name length.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>