link failures with bpf_map_xxx_elem_proto calls from net/core/filiter.c

[Or Gerlitz <ogerlitz@mellanox.com>]

Hi Daniel,

With net-next (and probably net or Linus tree too), I am getting these 
link failures

net/built-in.o:(.rodata+0x2210): undefined reference to 
`bpf_map_lookup_elem_proto'
net/built-in.o:(.rodata+0x2218): undefined reference to 
`bpf_map_update_elem_proto'
net/built-in.o:(.rodata+0x2220): undefined reference to 
`bpf_map_delete_elem_proto'

which are resolved when CONFIG_BPF_SYSCALL is added to the kernel 
config, I see now t hat this 4.0-rc1 commit d4052c4aea0cf "ebpf: remove 
CONFIG_BPF_SYSCALL ifdefs in socket filter code" removed the ifdefing, 
not sure what does "BPF internal header can deal with it" means.

Or.