bpf/scripts: Raise an exception if the correct number of sycalls are not generated

Currently the syscalls rst and subsequently man page are auto-generated
using function documentation present in bpf.h. If the documentation for the
syscall is missing or doesn't follow a specific format, then that syscall
is not dumped in the auto-generated rst.

This patch checks the number of syscalls documented within the header file
with those present as part of the enum bpf_cmd and raises an Exception if
they don't match. It is not needed with the currently documented upstream
syscalls, but can help in debugging when developing new syscalls when
there might be missing or misformatted documentation.

The function helper_number_check is moved to the Printer parent
class and renamed to elem_number_check as all the most derived children
classes are using this function now.

Signed-off-by: Usama Arif <usama.arif@bytedance.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20220119114442.1452088-3-usama.arif@bytedance.com