bpf: ensure all memory is initialized in bpf_get_current_comm

BPF helpers that take an ARG_PTR_TO_UNINIT_MEM must ensure that all of
the memory is set, including beyond the end of the string.

Signed-off-by: Barret Rhoden <brho@google.com>
Link: https://lore.kernel.org/r/20230407001808.1622968-1-brho@google.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>