apparmor: free the allocated pdb objects

policy_db objects are allocated with kzalloc() inside aa_alloc_pdb() and
are not cleared in the corresponding aa_free_pdb() function causing leak:

unreferenced object 0xffff88801f0a1400 (size 192):
  comm "apparmor_parser", pid 1247, jiffies 4295122827 (age 2306.399s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff81ddc612>] __kmem_cache_alloc_node+0x1e2/0x2d0
    [<ffffffff81c47c55>] kmalloc_trace+0x25/0xc0
    [<ffffffff83eb9a12>] aa_alloc_pdb+0x82/0x140
    [<ffffffff83ec4077>] unpack_pdb+0xc7/0x2700
    [<ffffffff83ec6b10>] unpack_profile+0x450/0x4960
    [<ffffffff83ecc129>] aa_unpack+0x309/0x15e0
    [<ffffffff83ebdb23>] aa_replace_profiles+0x213/0x33c0
    [<ffffffff83e8d341>] policy_update+0x261/0x370
    [<ffffffff83e8d66e>] profile_replace+0x20e/0x2a0
    [<ffffffff81eadfaf>] vfs_write+0x2af/0xe00
    [<ffffffff81eaf4c6>] ksys_write+0x126/0x250
    [<ffffffff890fa0b6>] do_syscall_64+0x46/0xf0
    [<ffffffff892000ea>] entry_SYSCALL_64_after_hwframe+0x6e/0x76

Free the pdbs inside aa_free_pdb(). While at it, rename the variable
representing an aa_policydb object to make the function more unified with
aa_pdb_free_kref() and aa_alloc_pdb().

Found by Linux Verification Center (linuxtesting.org).

Fixes: 98b824ff ("apparmor: refcount the pdb")
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Signed-off-by: John Johansen <john.johansen@canonical.com>