ravb: Fix use-after-free on `ifconfig eth0 down`

Commit a47b70ea ("ravb: unmap descriptors when freeing rings") has
introduced the issue seen in [1] reproduced on H3ULCB board.

Fix this by relocating the RX skb ringbuffer free operation, so that
swiotlb page unmapping can be done first. Freeing of aligned TX buffers
is not relevant to the issue seen in [1]. Still, reposition TX free
calls as well, to have all kfree() operations performed consistently
_after_ dma_unmap_*()/dma_free_*().

[1] Console screenshot with the problem reproduced:

salvator-x login: root
root@salvator-x:~# ifconfig eth0 up
Micrel KSZ9031 Gigabit PHY e6800000.ethernet-ffffffff:00: \
       attached PHY driver [Micrel KSZ9031 Gigabit PHY]   \
       (mii_bus:phy_addr=e6800000.ethernet-ffffffff:00, irq=235)
IPv6: ADDRCONF(NETDEV_UP): eth0: link is not ready
root@salvator-x:~#
root@salvator-x:~# ifconfig eth0 down

==================================================================
BUG: KASAN: use-after-free in swiotlb_tbl_unmap_single+0xc4/0x35c
Write of size 1538 at addr ffff8006d884f780 by task ifconfig/1649

CPU: 0 PID: 1649 Comm: ifconfig Not tainted 4.12.0-rc4-00004-g112eb07287d1 #32
Hardware name: Renesas H3ULCB board based on r8a7795 (DT)
Call trace:
[<ffff20000808f11c>] dump_backtrace+0x0/0x3a4
[<ffff20000808f4d4>] show_stack+0x14/0x1c
[<ffff20000865970c>] dump_stack+0xf8/0x150
[<ffff20000831f8b0>] print_address_description+0x7c/0x330
[<ffff200008320010>] kasan_report+0x2e0/0x2f4
[<ffff20000831eac0>] check_memory_region+0x20/0x14c
[<ffff20000831f054>] memcpy+0x48/0x68
[<ffff20000869ed50>] swiotlb_tbl_unmap_single+0xc4/0x35c
[<ffff20000869fcf4>] unmap_single+0x90/0xa4
[<ffff20000869fd14>] swiotlb_unmap_page+0xc/0x14
[<ffff2000080a2974>] __swiotlb_unmap_page+0xcc/0xe4
[<ffff2000088acdb8>] ravb_ring_free+0x514/0x870
[<ffff2000088b25dc>] ravb_close+0x288/0x36c
[<ffff200008aaf8c4>] __dev_close_many+0x14c/0x174
[<ffff200008aaf9b4>] __dev_close+0xc8/0x144
[<ffff200008ac2100>] __dev_change_flags+0xd8/0x194
[<ffff200008ac221c>] dev_change_flags+0x60/0xb0
[<ffff200008ba2dec>] devinet_ioctl+0x484/0x9d4
[<ffff200008ba7b78>] inet_ioctl+0x190/0x194
[<ffff200008a78c44>] sock_do_ioctl+0x78/0xa8
[<ffff200008a7a128>] sock_ioctl+0x110/0x3c4
[<ffff200008365a70>] vfs_ioctl+0x90/0xa0
[<ffff200008365dbc>] do_vfs_ioctl+0x148/0xc38
[<ffff2000083668f0>] SyS_ioctl+0x44/0x74
[<ffff200008083770>] el0_svc_naked+0x24/0x28

The buggy address belongs to the page:
page:ffff7e001b6213c0 count:0 mapcount:0 mapping:          (null) index:0x0
flags: 0x4000000000000000()
raw: 4000000000000000 0000000000000000 0000000000000000 00000000ffffffff
raw: 0000000000000000 ffff7e001b6213e0 0000000000000000 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8006d884f680: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8006d884f700: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff8006d884f780: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                   ^
 ffff8006d884f800: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8006d884f880: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================
Disabling lock debugging due to kernel taint
root@salvator-x:~#

Fixes: a47b70ea ("ravb: unmap descriptors when freeing rings")
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>