linuxkpi: Add rol32()

rol64() and rol32() are used by <linux/siphash.h>. The former was
added previously, before <linux/siphash.h> was added. However the latter
was not, and it broke the build on armv7.

Reported by: adrian
Reviewed by: adrian, rpokala
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54588