LinuxKPI: Rename a short description of the kmalloc type.

To avoid duplication in the vmstat -m output rename the kmalloc type short
description to 'lkpikmalloc' as the Linux emulation layer historically names
its linux malloc type as 'linux'.

Reviewed by: hselasky, kib, emaste
Differential Revision: https://reviews.freebsd.org/D30928
MFC after: 2 weeks