signal: cleanup sys_sigprocmask()

Cleanup. Remove the unneeded goto's, we can simply read blocked.sig[0]
unconditionally and then copy-to-user it if oset != NULL.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Matt Fleming <matt.fleming@linux.intel.com>