Audit: increase the maximum length of the key field

Key lengths were arbitrarily limited to 32 characters.  If userspace is going
to start using the single kernel key field as multiple virtual key fields
(example key=key1,key2,key3,key4) we should give them enough room to work.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>