dh key: fix rounding up KDF output length

Commit 383203ef

 ("dh key: get rid of stack allocated array") changed
kdf_ctr() to assume that the length of key material to derive is a
multiple of the digest size.  The length was supposed to be rounded up
accordingly.  However, the round_up() macro was used which only gives
the correct result on power-of-2 arguments, whereas not all hash
algorithms have power-of-2 digest sizes.  In some cases this resulted in
a write past the end of the 'outbuf' buffer.

Fix it by switching to roundup(), which works for non-power-of-2 inputs.

Reported-by:  <syzbot+486f97f892efeb2075a3@syzkaller.appspotmail.com>
Reported-by:  <syzbot+29d17b7898b41ee120a5@syzkaller.appspotmail.com>
Reported-by:  <syzbot+8a608baf8751184ec727@syzkaller.appspotmail.com>
Reported-by:  <syzbot+d04e58bd384f1fe0b112@syzkaller.appspotmail.com>
Fixes: 383203ef

 ("dh key: get rid of stack allocated array")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Tycho Andersen <tycho@tycho.ws>
Signed-off-by: James Morris <james.morris@microsoft.com>