dh key: get rid of stack allocated array

dh key: get rid of stack allocated array

We're interested in getting rid of all of the stack allocated arrays in the
kernel: https://lkml.org/lkml/2018/3/7/621

This particular vla is used as a temporary output buffer in case there is
too much hash output for the destination buffer. Instead, let's just
allocate a buffer that's big enough initially, but only copy back to
userspace the amount that was originally asked for.

v2: allocate enough in the original output buffer vs creating a temporary
    output buffer

Signed-off-by: Tycho Andersen <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
CC: David Howells <[email protected]>
CC: James Morris <[email protected]>
CC: "Serge E. Hallyn" <[email protected]>
CC: Eric Biggers <[email protected]>
Signed-off-by: James Morris <[email protected]>