bcachefs: Add a debug mode that always reads from every btree replica

bcachefs: Add a debug mode that always reads from every btree replica

There's a new module parameter, verify_all_btree_replicas, that enables
reading from every btree replica when reading in btree nodes and
comparing them against each other. We've been seeing some strange btree
corruption - this will hopefully aid in tracking it down and catching it
more often.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>