linuxkpi: Fix an off-by-one error in the kfifo implementation

"total" is the number of slots in the array, so wraparound needs to be
done when "first" or "last" is greater than or equal to the number of
slots.

Note that no consumers of the code are currently connected to the kernel
build.

Reported by: Stanislav Fort <stanislav.fort@aisle.com>
Reviewed by: bz, emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D56371