exec: Move the call of prepare_binprm into search_binary_handler

exec: Move the call of prepare_binprm into search_binary_handler

The code in prepare_binary_handler needs to be run every time
search_binary_handler is called so move the call into search_binary_handler
itself to make the code simpler and easier to understand.

Link: https://lkml.kernel.org/r/87d070zrvx.fsf_-_@x220.int.ebiederm.org
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: James Morris <jamorris@linux.microsoft.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>