bcachefs: Fix oob write in __bch2_btree_node_write

bcachefs: Fix oob write in __bch2_btree_node_write

Fix a possible out of bounds write in __bch2_btree_node_write when
the data buffer padding is cleared up to the block size. The out of
bounds write is possible if the data buffers size is not a multiple
of the block size.

Signed-off-by: Dan Robertson <dan@dlrobertson.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>