make it easier to catch those who try to modify ->d_name

make it easier to catch those who try to modify ->d_name

Turn d_name into an anon union of const struct qstr d_name with
struct qstr __d_name.  Very few places need to modify it (all
in fs/dcache.c); those are switched to use of ->__d_name.

Note that ->d_name can actually change under you unless you have
the right locking environment; this const just prohibits accidentally
doing stores without being easily spotted.

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