apparmor: fix wrong buffer allocation in aa_new_mount

Fix the following trace caused by the dev_path buffer not being
allocated.

[  641.044262] AppArmor WARN match_mnt: ((devpath && !devbuffer)):
[  641.044284] WARNING: CPU: 1 PID: 30709 at ../security/apparmor/mount.c:385 match_mnt+0x133/0x180
[  641.044286] Modules linked in: snd_hda_codec_generic ledtrig_audio snd_hda_intel snd_hda_codec snd_hda_core qxl ttm snd_hwdep snd_pcm drm_kms_helper snd_seq_midi snd_seq_midi_event drm snd_rawmidi crct10dif_pclmul crc32_pclmul ghash_clmulni_intel iptable_mangle aesni_intel aes_x86_64 xt_tcpudp crypto_simd snd_seq cryptd bridge stp llc iptable_filter glue_helper snd_seq_device snd_timer joydev input_leds snd serio_raw fb_sys_fops 9pnet_virtio 9pnet syscopyarea sysfillrect soundcore sysimgblt qemu_fw_cfg mac_hid sch_fq_codel parport_pc ppdev lp parport ip_tables x_tables autofs4 8139too psmouse 8139cp i2c_piix4 pata_acpi mii floppy
[  641.044318] CPU: 1 PID: 30709 Comm: mount Tainted: G      D W         5.1.0-rc4+ #223
[  641.044320] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[  641.044323] RIP: 0010:match_mnt+0x133/0x180
[  641.044325] Code: 41 5d 41 5e 41 5f c3 48 8b 4c 24 18 eb b1 48 c7 c6 08 84 26 83 48 c7 c7 f0 56 54 83 4c 89 54 24 08 48 89 14 24 e8 7d d3 bb ff <0f> 0b 4c 8b 54 24 08 48 8b 14 24 e9 25 ff ff ff 48 c7 c6 08 84 26
[  641.044327] RSP: 0018:ffffa9b34ac97d08 EFLAGS: 00010282
[  641.044329] RAX: 0000000000000000 RBX: ffff9a86725a8558 RCX: 0000000000000000
[  641.044331] RDX: 0000000000000002 RSI: 0000000000000001 RDI: 0000000000000246
[  641.044333] RBP: ffffa9b34ac97db0 R08: 0000000000000000 R09: 0000000000000000
[  641.044334] R10: 0000000000000000 R11: 00000000000077f5 R12: 0000000000000000
[  641.044336] R13: ffffa9b34ac97e98 R14: ffff9a865e000008 R15: ffff9a86c4cf42b8
[  641.044338] FS:  00007fab73969740(0000) GS:ffff9a86fbb00000(0000) knlGS:0000000000000000
[  641.044340] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  641.044342] CR2: 000055f90bc62035 CR3: 00000000aab5f006 CR4: 00000000003606e0
[  641.044346] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  641.044348] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  641.044349] Call Trace:
[  641.044355]  aa_new_mount+0x119/0x2c0
[  641.044363]  apparmor_sb_mount+0xd4/0x430
[  641.044367]  security_sb_mount+0x46/0x70
[  641.044372]  do_mount+0xbb/0xeb0
[  641.044377]  ? memdup_user+0x4b/0x70
[  641.044380]  ksys_mount+0x7e/0xd0
[  641.044384]  __x64_sys_mount+0x21/0x30
[  641.044388]  do_syscall_64+0x5a/0x1a0
[  641.044392]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  641.044394] RIP: 0033:0x7fab73a8790a
[  641.044397] Code: 48 8b 0d 89 85 0c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 56 85 0c 00 f7 d8 64 89 01 48
[  641.044399] RSP: 002b:00007ffe0ffe4238 EFLAGS: 00000206 ORIG_RAX: 00000000000000a5
[  641.044401] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fab73a8790a
[  641.044429] RDX: 000055f90bc6203b RSI: 00007ffe0ffe57b1 RDI: 00007ffe0ffe57a5
[  641.044431] RBP: 00007ffe0ffe4250 R08: 0000000000000000 R09: 00007fab73b51d80
[  641.044433] R10: 00000000c0ed0004 R11: 0000000000000206 R12: 000055f90bc610b0
[  641.044434] R13: 00007ffe0ffe4330 R14: 0000000000000000 R15: 0000000000000000
[  641.044457] irq event stamp: 0
[  641.044460] hardirqs last  enabled at (0): [<0000000000000000>]           (null)
[  641.044463] hardirqs last disabled at (0): [<ffffffff82290114>] copy_process.part.30+0x734/0x23f0
[  641.044467] softirqs last  enabled at (0): [<ffffffff82290114>] copy_process.part.30+0x734/0x23f0
[  641.044469] softirqs last disabled at (0): [<0000000000000000>]           (null)
[  641.044470] ---[ end trace c0d54bdacf6af6b2 ]---

Fixes: df323337 ("apparmor: Use a memory pool instead per-CPU caches")
Signed-off-by: John Johansen <john.johansen@canonical.com>