bcachefs: fix buffer overflow in nocow write path

BCH_REPLICAS_MAX isn't the actual maximum number of pointers in an
extent, it's the maximum number of dirty pointers.

We don't have a real restriction on the number of cached pointers, and
we don't want a fixed size array here anyways - so switch to
DARRAY_PREALLOCATED().

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Reported-and-tested-by: Daniel J Blueman <daniel@quora.org>