apparmor: validate DFA start states are in bounds in unpack_pdb

In the Linux kernel, the following vulnerability has been resolved:

apparmor: validate DFA start states are in bounds in unpack_pdb

Start states are read from untrusted data and used as indexes into the

DFA state tables. The aa_dfa_next() function call in unpack_pdb() will

access dfa->tables[YYTD_ID_BASE][start], and if the start state exceeds

the number of states in the DFA, this results in an out-of-bound read.

==================================================================

BUG: KASAN: slab-out-of-bounds in aa_dfa_next+0x2a1/0x360

Read of size 4 at addr ffff88811956fb90 by task su/1097

...

Reject policies with out-of-bounds start states during unpacking

to prevent the issue.