core: add BPF LSM functions

core: add BPF LSM functions

This adds 6 functions to implement RestrictFileSystems=

* lsm_bpf_supported() checks if LSM BPF is supported. It checks that
  cgroupv2 is used, that BPF LSM is enabled, and tries to load the BPF
  LSM program which makes sure BTF and hash of maps are supported, and
  BPF LSM programs can be loaded.
* lsm_bpf_setup() loads and attaches the LSM BPF program.
* lsm_bpf_unit_restrict_filesystems() populates the hash of maps BPF map with the
  cgroupID and the set of allowed or denied filesystems.
* lsm_bpf_cleanup() removes a cgroupID entry from the hash of maps.
* lsm_bpf_map_restrict_fs_fd() is a helper function to get the file
  descriptor of the BPF map.
* lsm_bpf_destroy() is a wrapper around the destroy function of the BPF
  skeleton file.