signals: sys_ssetmask() uses uninitialized newmask

Commit 77097ae5 ("most of set_current_blocked() callers want
SIGKILL/SIGSTOP removed from set") removed the initialization of newmask
by accident, causing ltp to complain like this:

  ssetmask01    1  TFAIL  :  sgetmask() failed: TEST_ERRNO=???(0): Success

Restore the proper initialization.

Reported-and-tested-by: CAI Qian <caiqian@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: stable@kernel.org	# v3.5+
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>