libbpf: Avoid allocating reg_name with sscanf in parse_usdt_arg()

The reg_name in parse_usdt_arg() is used to hold register name, which
is short enough to be held in a 16-byte array, so we could define
reg_name as char reg_name[16] to avoid dynamically allocating reg_name
with sscanf.

Suggested-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20221018145538.2046842-1-xukuohai@huaweicloud.com