crypto: testmgr - Dynamically allocate xbuf and axbuf

We currently allocate temporary memory that is used for testing
statically.  This renders the testing engine non-reentrant. As
algorithms may nest, i.e., one may construct another in order to
carry out a part of its operation, this is unacceptable.  For
example, it has been reported that an AEAD implementation allocates
a cipher in its setkey function, which causes it to fail during
testing as the temporary memory is overwritten.

This patch replaces the static memory with dynamically allocated
buffers.  We need a maximum of 16 pages so this slightly increases
the chances of an algorithm failing due to memory shortage.
However, as testing usually occurs at registration, this shouldn't
be a big problem.

Reported-by: Shasi Pulijala <spulijala@amcc.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>