We normally avoid having too many dirty keys in the btree key cache, to
ensure that we can always shrink our caches to reclaim memory if needed.

But this check was causing us to go into an infinite loop on startup, in
the btree insert path before journal reclaim was started.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>