bcache: add code comments for bch_btree_node_get() and __bch_btree_node_alloc()

This patch adds code comments to bch_btree_node_get() and
__bch_btree_node_alloc() that NULL pointer will not be returned and it
is unnecessary to check NULL pointer by the callers of these routines.

Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20231120052503.6122-10-colyli@suse.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>