UBUNTU: ubuntu-host: add generic lseek op

UBUNTU: ubuntu-host: add generic lseek op

BugLink: https://bugs.launchpad.net/bugs/1935807
BugLink: https://bugs.launchpad.net/bugs/1934110
Currently the lseek op is null and seeking on the esm-token procfs
file will cause an oops as detected by the stress-ng procfs stressor
as shown by the following splat:

[  942.470568] BUG: kernel NULL pointer dereference, address: 0000000000000000
[  942.471157] #PF: supervisor instruction fetch in kernel mode
[  942.471724] #PF: error_code(0x0010) - not-present page
[  942.472297] PGD 0 P4D 0
[  942.472867] Oops: 0010 [#1] SMP PTI
[  942.473435] CPU: 2 PID: 5661 Comm: stress-ng Not tainted 5.13.0-9-generic #9
[  942.474012] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
[  942.474599] RIP: 0010:0x0
[  942.475194] Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
[  942.475792] RSP: 0018:ffffaacf80ff7eb8 EFLAGS: 00010246
[  942.476383] RAX: 0000000000000000 RBX: ffff9eaa8a175240 RCX: 0000000000000001
[  942.476986] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff9eaa838d5800
[  942.477600] RBP: ffffaacf80ff7ed0 R08: 0000000000004000 R09: 0000000000000004
[  942.478203] R10: 0000000000000002 R11: 0000000000000000 R12: 0000000000000000
[  942.478800] R13: 0000000000000000 R14: ffffffffffffffea R15: ffff9eaa838d5800
[  942.479399] FS:  00007f998d487f00(0000) GS:ffff9eaaffc80000(0000) knlGS:0000000000000000
[  942.480006] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  942.480607] CR2: ffffffffffffffd6 CR3: 000000010a774002 CR4: 0000000000370ee0
[  942.481219] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  942.481855] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  942.482458] Call Trace:
[  942.483052]  proc_reg_llseek+0x4e/0x80
[  942.483646]  ? __fdget_pos+0x43/0x50
[  942.484234]  ksys_lseek+0x84/0xc0
[  942.484815]  __x64_sys_lseek+0x1a/0x20
[  942.485412]  do_syscall_64+0x61/0xb0
[  942.485966]  ? asm_exc_page_fault+0x8/0x30
[  942.486476]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[  942.486994] RIP: 0033:0x7f998d5c1cdb
[  942.487512] Code: ff ff c3 0f 1f 40 00 48 8b 15 89 81 0d 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb ba 0f 1f 00 f3 0f 1e fa b8 08 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 05 c3 0f 1f 40 00 48 8b 15 59 81 0d 00 f7 d8
[  942.488593] RSP: 002b:00007ffdf61c5328 EFLAGS: 00000246 ORIG_RAX: 0000000000000008
[  942.489148] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f998d5c1cdb
[  942.489710] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000004
[  942.490252] RBP: 0000000000000004 R08: 000001785e4740dd R09: 0000562dbebb9e50
[  942.490801] R10: 00007ffdf61c5300 R11: 0000000000000246 R12: 00007ffdf61c63f0
[  942.491354] R13: 00007ffdf61c53f0 R14: 0000000000000003 R15: 00000000000001e9

Fix this by using the generic lseek operation.

Fixes: 71adefd254f2 ("UBUNTU: Add ubuntu-host module")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>