kallsyms: ignore __kstrtab_* and __kstrtabns_* symbols

Every EXPORT_SYMBOL creates __kstrtab_* and __kstrtabns_*, which
consumes 15-20% of the kallsyms entries.

For example, on the system built from the x86_64 defconfig,

  $ cat /proc/kallsyms | wc
     129527    388581   5685465
  $ cat /proc/kallsyms | grep __kstrtab | wc
      23489     70467   1187932

We already ignore __crc_* symbols populated by EXPORT_SYMBOL, so it
should be fine to ignore __kstrtab_* and __kstrtabns_* as well.

This makes vmlinux a bit smaller.

  $ size vmlinux.before vmlinux.after
     text    data     bss     dec     hex filename
  22785374        8559694 1413328 32758396        1f3da7c vmlinux.before
  22785374        8137806 1413328 32336508        1ed6a7c vmlinux.after

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>