bcachefs: Second layer of refcounting for new stripes

bcachefs: Second layer of refcounting for new stripes

This will be used for move writes, which will be waiting until the
stripe is created to do the index update. They need to prevent the
stripe from being reclaimed until their index update is done, so we need
another refcount that just keeps the stripe open.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
# Conflicts:
#	fs/bcachefs/ec.c
#	fs/bcachefs/io.c