LinuxKPI: add UUID_STRING_LEN and GUID_INIT to uuid.h

Add a definition for UUID_STRING_LEN to uuid.h as needed by a driver.
Also add GUID_INIT for drm-kmod [1].

Submitted by: wulf [1]
Reviewed by: hselasky (earlier), wulf
Differential Revision: https://reviews.freebsd.org/D34235

(cherry picked from commit 97009980c492f5bc44a942cee09ac1ccfe4ae00a)