INIT_SIGHAND: use SIG_DFL instead of NULL

Cosmetic, no changes in the compiled code. Just s/NULL/SIG_DFL/ to make
it more readable and grep-friendly.

Note: probably SIG_IGN makes more sense, we could kill ignore_signals().
But then kernel_init() should do flush_signal_handlers() before exec().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Cedric Le Goater <clg@fr.ibm.com>
Cc: Dave Hansen <haveblue@us.ibm.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Herbert Poetzl <herbert@13thfloor.at>
Cc: Mathias Krause <Mathias.Krause@secunet.com>
Acked-by: Roland McGrath <roland@redhat.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Cc: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/init_task.h

@@ -35,7 +35,7 @@ extern struct nsproxy init_nsproxy;
 
 #define INIT_SIGHAND(sighand) {						\
 	.count		= ATOMIC_INIT(1), 				\
-	.action		= { { { .sa_handler = NULL, } }, },		\
+	.action		= { { { .sa_handler = SIG_DFL, } }, },		\
 	.siglock	= __SPIN_LOCK_UNLOCKED(sighand.siglock),	\
 	.signalfd_wqh	= __WAIT_QUEUE_HEAD_INITIALIZER(sighand.signalfd_wqh),	\
 }