When creating a new stripe, we may reuse an existing stripe that has
some empty and some nonempty blocks.

Generally, the existing stripe won't change underneath us - except for
block sector counts, which we copy to the new key in
ec_stripe_key_update.

But the device removal path can now invalidate stripe pointers to a
device, and that can race with stripe reuse.

Change ec_stripe_key_update() to check for and resolve this
inconsistency.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>