sched.h: drop in_ubsan field when UBSAN is in trap mode

in_ubsan field of task_struct is only used in lib/ubsan.c, which in its
turn is used only `ifneq ($(CONFIG_UBSAN_TRAP),y)`.

Removing unnecessary field from a task_struct will help preserve the ABI
between vanilla and CONFIG_UBSAN_TRAP'ed kernels.  In particular, this
will help enabling bounds sanitizer transparently for Android's GKI.

Signed-off-by: Elena Petrova <lenaptr@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Jann Horn <jannh@google.com>
Link: https://lkml.kernel.org/r/20200910134802.3160311-1-lenaptr@google.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/sched.h

@@ -1013,7 +1013,7 @@ struct task_struct {
 	struct held_lock		held_locks[MAX_LOCK_DEPTH];
 #endif
 
-#ifdef CONFIG_UBSAN
+#if defined(CONFIG_UBSAN) && !defined(CONFIG_UBSAN_TRAP)
 	unsigned int			in_ubsan;
 #endif