sunrpc/cache.h: fix coding style

Neaten code style in get_int().
Also use sizeof() instead of hard coded number as suggested by
Joe Perches <joe@perches.com>.

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