[CRYPTO] chainiv: Avoid lock spinning where possible

This patch makes chainiv avoid spinning by postponing requests on lock
contention if the user allows the use of asynchronous algorithms.  If
a synchronous algorithm is requested then we behave as before.

This should improve IPsec performance on SMP when two CPUs attempt to
transmit over the same SA.  Currently one of them will spin doing nothing
waiting for the other CPU to finish its encryption.  This patch makes it
postpone the request and get on with other work.

If only one CPU is transmitting for a given SA, then we will process
the request synchronously as before.

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