new fields in struct inode - i_cdev and i_cindex.  When we do open() on
a character device we cache result of cdev lookup in inode and put the
inode on a cyclic list anchored in cdev.  If we already have that done,
we don't bother with any lookups.  When inode disappears it's removed
from the list.  When cdev gets unregistered we remove all cached
references to it (and remove such inodes from the list).  cdev is held
until final fput() now.