rds: drop "select LLIST"

Commit 1bc144b6 ("net, rds, Replace xlist in net/rds/xlist.h with
llist") added "select LLIST" to the RDS_RDMA Kconfig entry. But there is
no Kconfig symbol named LLIST. The select statement for that symbol is a
nop. Drop it.

lib/llist.o is builtin, so all that's needed to use the llist
functionality is to include linux/llist.h, which this commit also did.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>