RDMA/hns: Work around broken constant propagation in gcc 8

gcc 8.3 and 5.4 throw this:

In function 'modify_qp_init_to_rtr',
././include/linux/compiler_types.h:322:38: error: call to '__compiletime_assert_1859' declared with attribute error: FIELD_PREP: value too large for the field
  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
[..]
drivers/infiniband/hw/hns/hns_roce_common.h:91:52: note: in expansion of macro 'FIELD_PREP'
   *((__le32 *)ptr + (field_h) / 32) |= cpu_to_le32(FIELD_PREP(   \
                                                    ^~~~~~~~~~
drivers/infiniband/hw/hns/hns_roce_common.h:95:39: note: in expansion of macro '_hr_reg_write'
 #define hr_reg_write(ptr, field, val) _hr_reg_write(ptr, field, val)
                                       ^~~~~~~~~~~~~
drivers/infiniband/hw/hns/hns_roce_hw_v2.c:4412:2: note: in expansion of macro 'hr_reg_write'
  hr_reg_write(context, QPC_LP_PKTN_INI, lp_pktn_ini);

Because gcc has miscalculated the constantness of lp_pktn_ini:

	mtu = ib_mtu_enum_to_int(ib_mtu);
	if (WARN_ON(mtu < 0)) [..]
	lp_pktn_ini = ilog2(MAX_LP_MSG_LEN / mtu);

Since mtu is limited to {256,512,1024,2048,4096} lp_pktn_ini is between 4
and 8 which is compatible with the 4 bit field in the FIELD_PREP.

Work around this broken compiler by adding a 'can never be true'
constraint on lp_pktn_ini's value which clears out the problem.

Fixes: f0cb411a ("RDMA/hns: Use new interface to modify QP context")
Link: https://lore.kernel.org/r/0-v1-c773ecb137bc+11f-hns_gcc8_jgg@nvidia.comReported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/hw/hns/hns_roce_hw_v2.c

@@ -4397,7 +4397,12 @@ static int modify_qp_init_to_rtr(struct ib_qp *ibqp,
 	hr_qp->path_mtu = ib_mtu;
 
 	mtu = ib_mtu_enum_to_int(ib_mtu);
-	if (WARN_ON(mtu < 0))
+	if (WARN_ON(mtu <= 0))
+		return -EINVAL;
+#define MAX_LP_MSG_LEN 65536
+	/* MTU * (2 ^ LP_PKTN_INI) shouldn't be bigger than 64KB */
+	lp_pktn_ini = ilog2(MAX_LP_MSG_LEN / mtu);
+	if (WARN_ON(lp_pktn_ini >= 0xF))
 		return -EINVAL;
 
 	if (attr_mask & IB_QP_PATH_MTU) {
@@ -4405,10 +4410,6 @@ static int modify_qp_init_to_rtr(struct ib_qp *ibqp,
 		hr_reg_clear(qpc_mask, QPC_MTU);
 	}
 
-#define MAX_LP_MSG_LEN 65536
-	/* MTU * (2 ^ LP_PKTN_INI) shouldn't be bigger than 64KB */
-	lp_pktn_ini = ilog2(MAX_LP_MSG_LEN / mtu);
-
 	hr_reg_write(context, QPC_LP_PKTN_INI, lp_pktn_ini);
 	hr_reg_clear(qpc_mask, QPC_LP_PKTN_INI);