bpftool: Do not check return value from libbpf_set_strict_mode()

The function always returns 0, so we don't need to check whether the
return value is 0 or not.

This change was first introduced in commit a777e18f ("bpftool: Use
libbpf 1.0 API mode instead of RLIMIT_MEMLOCK"), but later reverted to
restore the unconditional rlimit bump in bpftool. Let's re-add it.

Co-developed-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220610112648.29695-3-quentin@isovalent.com