smc: add support for splice()

Provide an implementation for splice() when we are using SMC. See
smc_splice_read() for further details.

Signed-off-by: Stefan Raspl <raspl@linux.ibm.com>
Signed-off-by: Ursula Braun <ubraun@linux.ibm.com&gt;&lt;>
Signed-off-by: David S. Miller <davem@davemloft.net>