No functional changes, preparation to simplify the review of the next change.

1. We can read current->block lockless, nobody else can ever change this mask.

2. Calculate the resulting sigset_t outside of ->siglock into the temporary
   variable, then take ->siglock and change ->blocked.

Also, kill the stale comment about BKL.
Signed-off-by: NOleg Nesterov <oleg@redhat.com>
Reviewed-by: NMatt Fleming <matt.fleming@linux.intel.com>
Acked-by: NTejun Heo <tj@kernel.org>