fs/affs: add validation block function

Avoid repeating 4 times the same calculation.

Link: http://lkml.kernel.org/r/20170109191208.6085-3-fabf@skynet.be


Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>