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]
MFC after:	3 days
Reviewed by:	hselasky (earlier), wulf
Differential Revision: https://reviews.freebsd.org/D34235

sys/compat/linuxkpi/common/include/linux/uuid.h

@@ -33,6 +33,28 @@
 #ifndef	_LINUXKPI_LINUX_UUID_H
 #define	_LINUXKPI_LINUX_UUID_H
 
+#define	UUID_STRING_LEN	36
+
+#define	GUID_INIT(x0_3, x4_5, x6_7, x8, x9, x10, x11, x12, x13, x14, x15) \
+	((guid_t) { .x = { 						\
+		[0]  =  (x0_3)        & 0xff,				\
+		[1]  = ((x0_3) >> 8)  & 0xff,				\
+		[2]  = ((x0_3) >> 16) & 0xff,				\
+		[3]  = ((x0_3) >> 24) & 0xff,				\
+		[4]  =  (x4_5)        & 0xff,				\
+		[5]  = ((x4_5) >> 8)  & 0xff,				\
+		[6]  =  (x6_7)        & 0xff,				\
+		[7]  = ((x6_7) >> 8)  & 0xff,				\
+		[8]  =  (x8),						\
+		[9]  =  (x9),						\
+		[10] =  (x10),						\
+		[11] =  (x11),						\
+		[12] =  (x12),						\
+		[13] =  (x13),						\
+		[14] =  (x14),						\
+		[15] =  (x15)						\
+}})
+
 typedef struct {
 	char	x[16];
 } guid_t;