compat: add in_compat_syscall to ask whether we're in a compat syscall

A lot of code currently abuses is_compat_task to determine this.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: Clemens Ladisch <clemens@ladisch.de>
Cc: David Airlie <airlied@linux.ie>
Cc: David Herrmann <dh.herrmann@googlemail.com>
Cc: David Miller <davem@davemloft.net>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Eric Paris <eparis@redhat.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Oded Gabbay <oded.gabbay@gmail.com>
Cc: Oleg Drokin <oleg.drokin@intel.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>