bpf: Fix bpf_prog_pack build HPAGE_PMD_SIZE

Fix build with CONFIG_TRANSPARENT_HUGEPAGE=n with BPF_PROG_PACK_SIZE as
PAGE_SIZE.

Fixes: 57631054 ("bpf: Introduce bpf_prog_pack allocator")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220208220509.4180389-3-song@kernel.org