LinuxKPI: cast jiffies_to_msecs() result

jiffies_to_msecs() is expected to return an uint_t. In order to avoid
changing printf format strings, properly cast the result to the
expected type.

Sponsored by: The FreeBSD Foundation
MFC after: 10 days
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D40486