fold _d_rehash() and __d_rehash() together

The only place where we feed to __d_rehash() something other than
d_hash(dentry->d_name.hash) is __d_move(), where we give it d_hash
of another dentry.  Postpone rehashing until we'd switched the
names and we are rid of that exception, along with the need to
keep _d_rehash() and __d_rehash() separate.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>