exec: Fix error handling in begin_new_exec()

exec: Fix error handling in begin_new_exec()

commit 84c39ec57d409e803a9bb6e4e85daf1243e0e80b upstream.

If get_unused_fd_flags() fails, the error handling is incomplete because
bprm->cred is already set to NULL, and therefore free_bprm will not
unlock the cred_guard_mutex. Note there are two error conditions which
end up here, one before and one after bprm->cred is cleared.

Fixes: b8a61c9e7b4a ("exec: Generic execfd support")
Signed-off-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Acked-by: Eric W. Biederman <ebiederm@xmission.com>
Link: https://lore.kernel.org/r/AS8P193MB128517ADB5EFF29E04389EDAE4752@AS8P193MB1285.EURP193.PROD.OUTLOOK.COM
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>