sysctl: remove impossible condition check

Remove checks for conditions that will never happen. If procname is NULL
the loop would already had bailed out, so there's no need to check it
again.

At the same time this also compacts the function find_in_table() by
refactoring it to be easier to read.

Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
Reviewed-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>