bcachefs: bch2_ec_stripe_head_get() now checks for change in rw devices

bcachefs: bch2_ec_stripe_head_get() now checks for change in rw devices

This factors out ec_strie_head_devs_update(), which initializes the
bitmap of devices we're allocating from, and runs it every time
c->rw_devs_change_count changes.

We also cancel pending, not allocated stripes, since they may refer to
devices that are no longer available.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>