Linux: Fix octal detection in define_ddi_strtox()

Clang Tidy reported this as a misc-redundant-expression because writing
8 instead of '8' meant that the condition could never be true.

The only place where we have a chance of this being a bug would be in
nvlist_lookup_nvpair_ei_sep(). I am not sure if we ever pass an octal to
that, but if we ever do, it should work properly now instead of failing.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14575