crypto: acomp - fix wrong pointer stored by acomp_save_req()

acomp_save_req() stores &req->chain in req->base.data. When
acomp_reqchain_done() is invoked on asynchronous completion, it receives
&req->chain as the data argument but casts it directly to struct
acomp_req. Since data points to the chain member, all subsequent field
accesses are at a wrong offset, resulting in memory corruption.

The issue occurs when an asynchronous hardware implementation, such as
the QAT driver, completes a request that uses the DMA virtual address
interface (e.g. acomp_request_set_src_dma()). This combination causes
crypto_acomp_compress() to enter the acomp_do_req_chain() path, which
sets acomp_reqchain_done() as the completion callback via
acomp_save_req().

With KASAN enabled, this manifests as a general protection fault in
acomp_reqchain_done():

  general protection fault, probably for non-canonical address 0xe000040000000000
  KASAN: probably user-memory-access in range [0x0000400000000000-0x0000400000000007]
  RIP: 0010:acomp_reqchain_done+0x15b/0x4e0
  Call Trace:
   <IRQ>
   qat_comp_alg_callback+0x5d/0xa0 [intel_qat]
   adf_ring_response_handler+0x376/0x8b0 [intel_qat]
   adf_response_handler+0x60/0x170 [intel_qat]
   tasklet_action_common+0x223/0x820
   handle_softirqs+0x1ab/0x640
   </IRQ>

Fix this by storing the request itself in req->base.data instead of
&req->chain, so that acomp_reqchain_done() receives the correct pointer.
Simplify acomp_restore_req() accordingly to access req->chain directly.

Fixes: 64929fe8 ("crypto: acomp - Remove request chaining")
Cc: stable@vger.kernel.org
Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>