IB/srp: RHEL 6.5 build fix

Avoid that compilation against the 2.6.32-431.3.1.el6 kernel headers
fails with the following error message:

In file included from drivers/infiniband/ulp/srp/ib_srp.h:44,
                 from drivers/infiniband/ulp/srp/ib_srp.c:58:
include/rdma/ib_verbs.h:2126: error: redefinition of 'ib_inc_rkey'
drivers/infiniband/ulp/srp/../../../../include/linux/backport.h:71: note: previous definition of 'ib_inc_rkey' was here
IB/srp: RHEL 5.x build fix

Avoid that compilation fails on RHEL 5.x systems with the following
error message:

drivers/infiniband/ulp/srp/ib_srp.c:952: error: 'struct request' has no member named 'cmd_flags'
v2.0.18