Grr...  When loop in get_sb_bdev() had been switched from
global list of superblock to per-type one, we should have switched
from sb_entry(p) (aka. list_entry(p, struct super_block, s_list)) to
list_entry(p, struct super_block, s_instances).

	As it is, we end up with false negatives all the time.  I.e.
second mount from the same block device with the same type gices
a new superblock.  With obvious nasty results...

	This fixes that.