random: handle creditable entropy from atomic process context

The entropy accounting changes a static key when the RNG has
initialized, since it only ever initializes once. Static key changes,
however, cannot be made from atomic context, so depending on where the
last creditable entropy comes from, the static key change might need to
be deferred to a worker.

Previously the code used the execute_in_process_context() helper
function, which accounts for whether or not the caller is
in_interrupt(). However, that doesn't account for the case where the
caller is actually in process context but is holding a spinlock.

This turned out to be the case with input_handle_event() in
drivers/input/input.c contributing entropy:

  [<ffffffd613025ba0>] die+0xa8/0x2fc
  [<ffffffd613027428>] bug_handler+0x44/0xec
  [<ffffffd613016964>] brk_handler+0x90/0x144
  [<ffffffd613041e58>] do_debug_exception+0xa0/0x148
  [<ffffffd61400c208>] el1_dbg+0x60/0x7c
  [<ffffffd61400c000>] el1h_64_sync_handler+0x38/0x90
  [<ffffffd613011294>] el1h_64_sync+0x64/0x6c
  [<ffffffd613102d88>] __might_resched+0x1fc/0x2e8
  [<ffffffd613102b54>] __might_sleep+0x44/0x7c
  [<ffffffd6130b6eac>] cpus_read_lock+0x1c/0xec
  [<ffffffd6132c2820>] static_key_enable+0x14/0x38
  [<ffffffd61400ac08>] crng_set_ready+0x14/0x28
  [<ffffffd6130df4dc>] execute_in_process_context+0xb8/0xf8
  [<ffffffd61400ab30>] _credit_init_bits+0x118/0x1dc
  [<ffffffd6138580c8>] add_timer_randomness+0x264/0x270
  [<ffffffd613857e54>] add_input_randomness+0x38/0x48
  [<ffffffd613a80f94>] input_handle_event+0x2b8/0x490
  [<ffffffd613a81310>] input_event+0x6c/0x98

According to Guoyong, it's not really possible to refactor the various
drivers to never hold a spinlock there. And in_atomic() isn't reliable.

So, rather than trying to be too fancy, just punt the change in the
static key to a workqueue always. There's basically no drawback of doing
this, as the code already needed to account for the static key not
changing immediately, and given that it's just an optimization, there's
not exactly a hurry to change the static key right away, so deferal is
fine.

Reported-by: Guoyong Wang <guoyong.wang@mediatek.com>
Cc: stable@vger.kernel.org
Fixes: f5bda35f

 ("random: use static branch for crng_ready()")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>