infiniband: avoid overflow warning

A sockaddr_in structure on the stack getting passed into rdma_ip2gid
triggers this warning, since we memcpy into a larger sockaddr_in6
structure:

In function 'memcpy',
    inlined from 'rdma_ip2gid' at include/rdma/ib_addr.h:175:3,
    inlined from 'addr_event.isra.4.constprop' at drivers/infiniband/core/roce_gid_mgmt.c:693:2,
    inlined from 'inetaddr_event' at drivers/infiniband/core/roce_gid_mgmt.c:716:9:
include/linux/string.h:305:4: error: call to '__read_overflow2' declared with attribute error: detected read beyond size of object passed as 2nd parameter

The warning seems appropriate here, but the code is also clearly
correct, so we really just want to shut up this instance of the
output.

The best way I found so far is to avoid the memcpy() call and instead
replace it with a struct assignment.

Fixes: 6974f0c4 ("include/linux/string.h: add the option of fortified string.h functions")
Cc: Daniel Micay <danielmicay@gmail.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Doug Ledford <dledford@redhat.com>