correct the set of flags forbidden at d_set_d_op() time

DCACHE_OP_PRUNE in ->d_flags at the time of d_set_d_op() should've
been treated the same as any other DCACHE_OP_... - we forgot to adjust
that WARN_ON() when DCACHE_OP_PRUNE had been introduced...

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>