bcache: check return value from btree_node_alloc_replacement()

In btree_gc_rewrite_node(), pointer 'n' is not checked after it returns
from btree_gc_rewrite_node(). There is potential possibility that 'n' is
a non NULL ERR_PTR(), referencing such error code is not permitted in
following code. Therefore a return value checking is necessary after 'n'
is back from btree_node_alloc_replacement().

Signed-off-by: Coly Li <colyli@suse.de>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Cc:  <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20231120052503.6122-3-colyli@suse.de


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