- We weren't clearing the LRU btree
 - bch2_alloc_read() runs before bch2_check_alloc_key() deletes alloc
   keys for devices/buckets that don't exists, so it needs to check for
   that
 - bch2_check_lrus() needs to check that buckets exists
 - improve some error messages
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>