fix race in d_splice_alias()

rehashing the negative placeholder opens a race with d_lookup();
we unhash it almost immediately (by d_move()), but the race
window is there.  Since d_move() doesn't rely on target being
hashed, we don't need that d_rehash() at all.

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