Coverity gets a bit confused by loading fdt_size_dt_strings() and
using it in a memmove(). In fact this is safe because the callers
have verified this information (via FDT_RW_PROBE() in fdt_pack() or
construction in fdt_open_into()).
Passing in strings_size like we already do struct_size seems to get
Coverity to follow what's going on here.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>