bcachefs: memset bounce buffer portion to 0 after key_sort_fix_overlapping

Zero-initialize part of allocated bounce buffer which wasn't touched by
subsequent bch2_key_sort_fix_overlapping to mitigate later uinit-value
use KMSAN bug[1].

After applying the patch reproducer still triggers stack overflow[2] but
it seems unrelated to the uninit-value use warning. After further
investigation it was found that stack overflow occurs because KMSAN adds
too many function calls[3]. Backtrace of where the stack magic number gets
smashed was added as a reply to syzkaller thread[3].

It was confirmed that task's stack magic number gets smashed after the code
path where KSMAN detects uninit-value use is executed, so it can be assumed
that it doesn't contribute in any way to uninit-value use detection.

[1] https://syzkaller.appspot.com/bug?extid=6f655a60d3244d0c6718
[2] https://lore.kernel.org/lkml/66e57e46.050a0220.115905.0002.GAE@google.com
[3] https://lore.kernel.org/all/rVaWgPULej8K7HqMPNIu8kVNyXNjjCiTB-QBtItLFBmk0alH6fV2tk4joVPk97Evnuv4ZRDd8HB5uDCkiFG6u81xKdzDj-KrtIMJSlF6Kt8=@proton.me

Reported-by: syzbot+6f655a60d3244d0c6718@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6f655a60d3244d0c6718
Fixes: ec4edd7b ("bcachefs: Prep work for variable size btree node buffers")
Suggested-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Piotr Zalewski <pZ010001011111@proton.me>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_io.c

@@ -1195,6 +1195,10 @@ int bch2_btree_node_read_done(struct bch_fs *c, struct bch_dev *ca,
 	set_btree_bset(b, b->set, &b->data->keys);
 
 	b->nr = bch2_key_sort_fix_overlapping(c, &sorted->keys, iter);
+	memset((uint8_t *)(sorted + 1) + b->nr.live_u64s * sizeof(u64), 0,
+			btree_buf_bytes(b) -
+			sizeof(struct btree_node) -
+			b->nr.live_u64s * sizeof(u64));
 
 	u64s = le16_to_cpu(sorted->keys.u64s);
 	*sorted = *b->data;