libbpf: Fix potential multiplication overflow in mmap() size calculation

Prevent potential overflow performed in 32-bit integers, before assigning
result to size_t. Reported by LGTM static analysis.

Fixes: eba9c5f4 ("libbpf: Refactor global data map initialization")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200117060801.1311525-4-andriin@fb.com