libbpf: Fix unneeded extra initialization in bpf_map_batch_common

bpf_attr doesn't required to be declared with '= {}' as memset is used
in the code.

Fixes: 2ab3d86e ("libbpf: Add libbpf support to batch ops")
Reported-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Brian Vazquez <brianvv@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200116045918.75597-1-brianvv@google.com