More code cleanup.

Use constants to specify MAX_READ and MAX_WRITE buffer sizes and
use that to determine the size of the buffers that we need.

Be more careful about how the request header and data payload are
extracted.  For example, the old code did len -= hdr->len, but
since len == hdr->len, this value was always 0.  It turns out we
didn't use len thereafter, but we might want to for sanity checking
incoming requests.

Use const to make it clearer what data is coming out of the request.

Removed spurious error reply from FUSE_WRITE.  It serves no purpose
and is ignored by the kernel.

Bug: 6488845
Change-Id: Ia328532979868f0aaea43744a49662f2f4511bfe