[CRYPTO] hmac: Avoid calling virt_to_page on key

When HMAC gets a key longer than the block size of the hash, it needs
to feed it as input to the hash to reduce it to a fixed length.  As
it is HMAC converts the key to a scatter and gather list.  However,
this doesn't work on certain platforms if the key is not allocated
via kmalloc.  For example, the keys from tcrypt are stored in the
rodata section and this causes it to fail with HMAC on x86-64.

This patch fixes this by copying the key to memory obtained via
kmalloc before hashing it.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>