sunrpc/cache.h: replace simple_strtoul

This patch replaces the usage of simple_strtoul with kstrtoint in
get_int(), since the simple_str* family doesn't account for overflow
and is deprecated.
Also, in this specific case, the long from strtol is silently converted
to an int by the caller.

As Joe Perches <joe@perches.com> suggested, this patch also removes
the redundant temporary variable rv, since kstrtoint() will not write to
anint unless it's successful.

Cc: Joe Perches <joe@perches.com>
Signed-off-by: Eldad Zack <eldad@fogrefinery.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>