apparmor: fix possible memory leak in unpack_trans_table

If we fail to unpack the transition table then the table elements which
have been already allocated are not freed on error path.

unreferenced object 0xffff88802539e000 (size 128):
  comm "apparmor_parser", pid 903, jiffies 4294914938 (age 35.085s)
  hex dump (first 32 bytes):
    20 73 6f 6d 65 20 6e 61 73 74 79 20 73 74 72 69   some nasty stri
    6e 67 20 73 6f 6d 65 20 6e 61 73 74 79 20 73 74  ng some nasty st
  backtrace:
    [<ffffffff81ddb312>] __kmem_cache_alloc_node+0x1e2/0x2d0
    [<ffffffff81c47194>] __kmalloc_node_track_caller+0x54/0x170
    [<ffffffff81c225b9>] kmemdup+0x29/0x60
    [<ffffffff83e1ee65>] aa_unpack_strdup+0xe5/0x1b0
    [<ffffffff83e20808>] unpack_pdb+0xeb8/0x2700
    [<ffffffff83e23567>] unpack_profile+0x1507/0x4a30
    [<ffffffff83e27bfa>] aa_unpack+0x36a/0x1560
    [<ffffffff83e194c3>] aa_replace_profiles+0x213/0x33c0
    [<ffffffff83de9461>] policy_update+0x261/0x370
    [<ffffffff83de978e>] profile_replace+0x20e/0x2a0
    [<ffffffff81eac8bf>] vfs_write+0x2af/0xe00
    [<ffffffff81eaddd6>] ksys_write+0x126/0x250
    [<ffffffff88f34fb6>] do_syscall_64+0x46/0xf0
    [<ffffffff890000ea>] entry_SYSCALL_64_after_hwframe+0x6e/0x76

Call aa_free_str_table() on error path as was done before the blamed
commit. It implements all necessary checks, frees str_table if it is
available and nullifies the pointers.

Found by Linux Verification Center (linuxtesting.org).

Fixes: a0792e2c

 ("apparmor: make transition table unpack generic so it can be reused")
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Signed-off-by: John Johansen <john.johansen@canonical.com>