md: Use a larger buffer for the ident string

With the old size, the string could easily be truncated, resulting in
non-unique identifiers.

PR:		287679
Reported by:	Phil Krylov <phil@krylov.eu>
Reviewed by:	kib
MFC after:	2 weeks

sys/dev/md/md.c

@@ -256,7 +256,7 @@ struct md_s {
 	unsigned opencount;
 	unsigned fwheads;
 	unsigned fwsectors;
-	char ident[32];
+	char ident[DISK_IDENT_SIZE];
 	unsigned flags;
 	char name[20];
 	struct proc *procp;