ANDROID: timer: fix timer_setup with CFI

timer_setup casts the callback function to an incompatible type, which
trips CFI for all users of the API:

  CFI failure (target: [<ffffff8da5b6bc88>] nf_ct_frag6_expire+0x0/0x4):
  ...
  Call trace:
    __cfi_check_fail+0x1c/0x24
    ...
    call_timer_fn+0x304/0x308

Note that this only affects 4.14, the timer code was refactored in the
later upstream releases.

Bug: 130800382
Bug: 156058713
Change-Id: If7d706dcbe08ca0001d025606e5d3f3ca1559a7b
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>