bpf: fix pseudo calls for program loaded from ELF

bpf: fix pseudo calls for program loaded from ELF

clang 6.0 and onwards, for the external function call generates
BPF_PSEUDO_CALL instruction:
call pseudo +-off -> call another bpf function.
More details about that change: https://lwn.net/Articles/741773/
DPDK BPF implementation right now doesn't support multiple BPF
functions per module.
To overcome that problem, and preserve existing functionality
(ability to call allowed by user external functions),
bpf_elf_load() clears EBPF_PSEUDO_CALL value.
For details how to reproduce the issue:
https://bugs.dpdk.org/show_bug.cgi?id=259

Fixes: 5dba93ae5f2d ("bpf: add ability to load eBPF program from ELF object file")
Cc: stable@dpdk.org
Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>