libfdt: Tweak data handling to satisfy Coverity

In libfdt we often sanity test fdt_totalsize(fdt) fairly early, then
trust it (but *only* that header field) for the remainder of our work.
However, Coverity gets confused by this - it sees the byteswap in
fdt32_ld() and assumes that means it is coming from an untrusted source
everytime, resulting in many tainted data warnings.

Most of these end up with logic in fdt_get_string() as the unsafe
destination for this tainted data, so let's tweak the logic there to make
it clearer to Coverity that this is ok.

We add a sanity test on fdt_totalsize() to fdt_probe_ro_().  Because the
interface allows bare ints to be used for offsets, we already have the
assumption that totalsize must be 31-bits or less (2GiB would be a
ludicrously large fdt).  This makes this more explicit.

We also make fdt_probe_ro() return the size for convenience, and change the
logic in fdt_get_string() to keep it in a local so that Coverity can see
that it has already been bounds-checked.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>