bpf: add parenthesis around argument of BPF_LDST_BYTES()

BPF_LDST_BYTES() does not put it's argument in parenthesis
when referencing it.  This makes it impossible to pass pointers
obtained by address-of operator (e.g. BPF_LDST_BYTES(&insn)).
Add the parenthesis.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>