sunrpc/cache: convert to use string_escape_str()

There is nice kernel helper to escape a given strings by provided rules. Let's
use it instead of custom approach.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[bfields@redhat.com: fix length calculation]
Signed-off-by: J. Bruce Fields <bfields@redhat.com>