tools: bpftool: fix infinite loop in map create

"bpftool map create" has an infinite loop on "while (argc)". The error
case is missing.

Symptoms: when forgetting to type the keyword 'type' in front of 'hash':
$ sudo bpftool map create /sys/fs/bpf/dir/foobar hash key 8 value 8 entries 128
(infinite loop, taking all the CPU)
^C

After the patch:
$ sudo bpftool map create /sys/fs/bpf/dir/foobar hash key 8 value 8 entries 128
Error: unknown arg hash

Fixes: 0b592b5a ("tools: bpftool: add map create command")
Signed-off-by: Alban Crequy <alban@kinvolk.io>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Song Liu <songliubraving@fb.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>