zustr2stp.h: Add ZUSTR2STP() macro

zustr2stp.h: Add ZUSTR2STP() macro

It's a wrapper around zustr2stp() that calls SIZEOF_ARRAY() internally.
The function call is usually --in our code base, always-- called with an
array as the second argument.  For such an argument, one should call
SIZEOF_ARRAY().  To avoid mistakes, and simplify usage, let's add this
macro that does it internally.

BTW, this macro doesn't have any issues of double evaluation, because
sizeof() doesn't evaluate its argument (unless it's a VLA, but then
the static_assert(3) within SIZEOF_ARRAY() makes sure VLAs are not
allowed).

Cc: Christian Göttsche <cgzones@googlemail.com>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>