android/binder: use same_thread_group(proc->tsk, current) in binder_mmap()

With or without this change the checked condition can be falsely true if
proc->tsk execs, but this is fine: binder_alloc_mmap_handler() checks
vma->vm_mm == alloc->mm.

Link: https://lkml.kernel.org/r/aXY_uPYyUg4rwNOg@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Christan König <christian.koenig@amd.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Felix Kuehling <felix.kuehling@amd.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Simon Horman <horms@kernel.org>
Cc: Steven Price <steven.price@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: oleg-nesterov

drivers/android/binder.c

@@ -6015,7 +6015,7 @@ static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
 {
 	struct binder_proc *proc = filp->private_data;
 
-	if (proc->tsk != current->group_leader)
+	if (!same_thread_group(proc->tsk, current))
 		return -EINVAL;
 
 	binder_debug(BINDER_DEBUG_OPEN_CLOSE,