bcachefs: bch_member->seq

Add new fields for split brain detection:

 - bch_member->seq, which tracks the sequence number of the last superblock
   write that happened to each member device

 - bch_sb->write_time, which tracks the time of the last superblock write,
   to allow detection of when two members have diverged but had the same
   number of superblock writes.

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