Pull block driver updates from Jens Axboe:
 "Like the core side, not a lot of changes here, just two main items:

   - Series of patches (via Coly) with fixes for bcache (Coly,
     Christoph)

   - MD pull request from Song"

* tag 'for-5.6/drivers-2020-01-27' of git://git.kernel.dk/linux-block: (31 commits)
  bcache: reap from tail of c->btree_cache in bch_mca_scan()
  bcache: reap c->btree_cache_freeable from the tail in bch_mca_scan()
  bcache: remove member accessed from struct btree
  bcache: print written and keys in trace_bcache_btree_write
  bcache: avoid unnecessary btree nodes flushing in btree_flush_write()
  bcache: add code comments for state->pool in __btree_sort()
  lib: crc64: include <linux/crc64.h> for 'crc64_be'
  bcache: use read_cache_page_gfp to read the superblock
  bcache: store a pointer to the on-disk sb in the cache and cached_dev structures
  bcache: return a pointer to the on-disk sb from read_super
  bcache: transfer the sb_page reference to register_{bdev,cache}
  bcache: fix use-after-free in register_bcache()
  bcache: properly initialize 'path' and 'err' in register_bcache()
  bcache: rework error unwinding in register_bcache
  bcache: use a separate data structure for the on-disk super block
  bcache: cached_dev_free needs to put the sb page
  md/raid1: introduce wait_for_serialization
  md/raid1: use bucket based mechanism for IO serialization
  md: introduce a new struct for IO serialization
  md: don't destroy serial_info_pool if serialize_policy is true
  ...