modpost: abort if module name is too long

Module name has a limited length, but currently the build system
allows the build finishing even if the module name is too long.

  CC      /root/kprobe_example/abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz.mod.o
 /root/kprobe_example/abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz.mod.c:9:2:
 warning: initializer-string for array of chars is too long [enabled by default]
  .name = KBUILD_MODNAME,
  ^

but it's merely a warning.

This patch adds the check of the module name length in modpost and stops
the build properly.

Signed-off-by: Wanlong Gao <wanlong.gao@gmail.com>
Signed-off-by: Jessica Yu <jeyu@kernel.org>