sparc/compat: provide an accurate in_compat_syscall implementation

On sparc64 compat-enabled kernels, any task can make 32-bit and 64-bit
syscalls.  is_compat_task returns true in 32-bit tasks, which does not
necessarily imply that the current syscall is 32-bit.

Provide an in_compat_syscall implementation that checks whether the
current syscall is compat.

As far as I know, sparc is the only architecture on which is_compat_task
checks the compat status of the task and on which the compat status of a
syscall can differ from the compat status of the task.  On x86,
is_compat_task checks the syscall type, not the task type.

[akpm@linux-foundation.org: add comment, per Sam]
[akpm@linux-foundation.org: update comment, per Andy]
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: David S. Miller <davem@davemloft.net>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>