simple_lookup(): just set DCACHE_DONTCACHE

simple_lookup(): just set DCACHE_DONTCACHE

No need to mess with ->d_op at all.  Note that ->d_delete that always
returns 1 is equivalent to having DCACHE_DONTCACHE in ->d_flags.
Later the same thing will be placed into ->s_d_flags of the filesystems
where we want that behaviour for all dentries; then the check in
simple_lookup() will at least get unlikely() slapped on it.

NOTE: there are only two filesystems where
	* simple_lookup() might be called
	* default ->d_op is non-NULL
	* its ->d_delete() doesn't always return 1
If not for those, we could have simple_lookup() just set DCACHE_DONTCACHE
without even looking at ->d_op.  Filesystems in question are btrfs
and tracefs; both have ->d_delete() returning 1 on anything fed to
simple_lookup(), so both would be fine with simple_lookup() setting
DCACHE_DONTCACHE regardless of ->d_op.

IOW, we might want to drop the check for ->d_op in simple_lookup();
it's definitely a separate story, though.

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