security_dentry_init_security(): constify qstr argument

security_dentry_init_security(): constify qstr argument

Nothing outside of fs/dcache.c has any business modifying
dentry names; passing &dentry->d_name as an argument should
have that argument declared as a const pointer.

Acked-by: Casey Schaufler <casey@schaufler-ca.com> # smack part
Acked-by: Paul Moore <paul@paul-moore.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>