libbpf-rs: handle maps that were autocreate(false)

[eyakubovich]

If map auto creation was disabled, the Object would fail to load since the map's underlying fd will be -1.

This checks the if the map autocreate flag was disabled and skips creating a Map for it.

[danielocfb]

Hi @eyakubovich, thanks for the pull request! Would you be able to create a test case that illustrates the issue, please?

[eyakubovich]

@danielocfb certainly. Added test_sudo_map_autocreate_disable. With the test but without the patch:

failures:

---- test_sudo_map_autocreate_disable stdout ----
libbpf: loading /home/eyakubovich/libbpf-rs/libbpf-rs/tests/bin/map_auto_pin.bpf.o
libbpf: elf: section(3) license, size 4, link 0, flags 3, type=1
libbpf: license of /home/eyakubovich/libbpf-rs/libbpf-rs/tests/bin/map_auto_pin.bpf.o is GPL
libbpf: elf: section(4) .maps, size 40, link 0, flags 3, type=1
libbpf: elf: section(9) .BTF, size 431, link 0, flags 0, type=1
libbpf: elf: section(13) .symtab, size 456, link 1, flags 0, type=2
libbpf: looking for externs among 19 symbols...
libbpf: collected 0 externs total
libbpf: map 'auto_pin_map': at sec_idx 4, offset 0.
libbpf: map 'auto_pin_map': found type = 1.
libbpf: map 'auto_pin_map': found key_size = 4.
libbpf: map 'auto_pin_map': found value_size = 4.
libbpf: map 'auto_pin_map': found max_entries = 1.
libbpf: map 'auto_pin_map': found pinning = 1.
libbpf: map 'auto_pin_map': skipped auto-creating...
thread 'test_sudo_map_autocreate_disable' panicked at libbpf-rs/tests/test.rs:1654:10:
failed to load object: Error: Operation not permitted (os error 1)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

N.B. "Operation not permitted" happens because the fd is initialized to -1 and EPERM on Linux is 1. The code misinterprets the initialized FD for an error.

[danielocfb]

Looks good to me. Thanks for the fix!