9p/trans_virtio: use kvfree() for iov_iter_get_pages_alloc()

9p/trans_virtio: use kvfree() for iov_iter_get_pages_alloc()

The memory allocated by iov_iter_get_pages_alloc() can be allocated with
vmalloc() if kmalloc() failed -- see get_pages_array().

In that case we need to free it with vfree(), so let's use kvfree().

The bug manifests like this:

BUG: unable to handle kernel paging request at ffffeb0400072da0
IP: [<ffffffff8139c67b>] kfree+0x4b/0x140
PGD 0
Oops: 0000 [#1] PREEMPT SMP KASAN
CPU: 2 PID: 675 Comm: trinity-c2 Not tainted 4.7.0-rc7+ #14
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
task: ffff8800badef2c0 ti: ffff880069208000 task.ti: ffff880069208000
RIP: 0010:[<ffffffff8139c67b>]  [<ffffffff8139c67b>] kfree+0x4b/0x140
RSP: 0000:ffff88006920f3f0  EFLAGS: 00010282
RAX: ffffea0000000000 RBX: ffffc90001cb6000 RCX: 0000000000000000
RDX: 0000000000000001 RSI: 0000000000000246 RDI: ffffc90001cb6000
RBP: ffff88006920f410 R08: 0000000000000000 R09: dffffc0000000000
R10: ffff8800badefa30 R11: 0000056a3d3b0d9f R12: ffff88006920f620
R13: ffffeb0400072d80 R14: ffff8800baa94078 R15: 0000000000000000
FS:  00007fbd2b437700(0000) GS:ffff88011af00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffeb0400072da0 CR3: 000000006926d000 CR4: 00000000000006e0
Stack:
 0000000000000001 ffff88006920f620 ffffed001755280f ffff8800baa94078
 ffff88006920f6a8 ffffffff8310442b dffffc0000000000 ffff8800badefa30
 ffff8800badefa28 ffff88011af1fba0 1ffff1000d241e98 ffff8800ba892150
Call Trace:
 [<ffffffff8310442b>] p9_virtio_zc_request+0x72b/0xdb0
 [<ffffffff830f2116>] p9_client_zc_rpc.constprop.8+0x246/0xb10
 [<ffffffff830f5d79>] p9_client_read+0x4c9/0x750
 [<ffffffff8175ceac>] v9fs_fid_readpage+0x14c/0x320
 [<ffffffff8175d0b6>] v9fs_vfs_readpage+0x36/0x50
 [<ffffffff812c6f13>] filemap_fault+0x9a3/0xe60
 [<ffffffff81331878>] __do_fault+0x158/0x300
 [<ffffffff81339e01>] handle_mm_fault+0x1cf1/0x3c80
 [<ffffffff810c0aaa>] __do_page_fault+0x30a/0x8e0
 [<ffffffff810c10df>] do_page_fault+0x2f/0x80
 [<ffffffff810b5b07>] do_async_page_fault+0x27/0xa0
 [<ffffffff83296c48>] async_page_fault+0x28/0x30
Code: 00 80 41 54 53 49 01 fd 48 0f 42 05 b0 39 67 02 48 89 fb 49 01 c5 48 b8 00 00 00 00 00 ea ff ff 49 c1 ed 0c 49 c1 e5 06 49 01 c5 <49> 8b 45 20 48 8d 50 ff a8 01 4c 0f 45 ea 49 8b 55 20 48 8d 42
RIP  [<ffffffff8139c67b>] kfree+0x4b/0x140
 RSP <ffff88006920f3f0>
CR2: ffffeb0400072da0
---[ end trace f3d59a04bafec038 ]---

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>