xprtrdma: fix incorrect header size calculations

[ Upstream commit 912288442cb2f431bf3c8cb097a5de83bc6dbac1 ]

Currently the header size calculations are using an assignment
operator instead of a += operator when accumulating the header
size leading to incorrect sizes.  Fix this by using the correct
operator.

Addresses-Coverity: ("Unused value")
Fixes: 302d3deb ("xprtrdma: Prevent inline overflow")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>