fs/affs: use AFFS_MOUNT prefix for mount options

Currently, affs still uses direct access on mount_options.  This patch
prepares to use affs_clear/set/test_opt() like other filesystems.

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>