fsck_ffs/fsdb: fix -fno-common build

This one is also a small list:

- 3x duplicate definition (ufs2_zino, returntosingle, nflag)
- 5x 'needs extern', 3/5 of which are referenced in fsdb

-fno-common will become the default in GCC10/LLVM11.

MFC after:	1 week

sbin/fsck_ffs/fsck.h

@@ -127,7 +127,7 @@ struct inostat {
  * Inode state information is contained on per cylinder group lists
  * which are described by the following structure.
  */
-struct inostatlist {
+extern struct inostatlist {
 	long	il_numalloced;	/* number of inodes allocated in this cg */
 	struct inostat *il_stat;/* inostat info for this cylinder group */
 } *inostathead;
@@ -271,13 +271,13 @@ struct dups {
 	struct dups *next;
 	ufs2_daddr_t dup;
 };
-struct dups *duplist;		/* head of dup list */
-struct dups *muldup;		/* end of unique duplicate dup block numbers */
+extern struct dups *duplist;		/* head of dup list */
+extern struct dups *muldup;		/* end of unique duplicate dup block numbers */
 
 /*
  * Inode cache data structures.
  */
-struct inoinfo {
+extern struct inoinfo {
 	struct	inoinfo *i_nexthash;	/* next entry in hash chain */
 	ino_t	i_number;		/* inode number of this entry */
 	ino_t	i_parent;		/* inode number of parent */

sbin/fsck_ffs/gjournal.c

@@ -93,7 +93,6 @@ static LIST_HEAD(, cgchain) cglist = LIST_HEAD_INITIALIZER(cglist);
 static const char *devnam;
 static struct uufsd *diskp = NULL;
 static struct fs *fs = NULL;
-struct ufs2_dinode ufs2_zino;
 
 static void putcgs(void);
 

sbin/fsck_ffs/globs.c

@@ -117,6 +117,10 @@ volatile sig_atomic_t	got_sigalarm;	/* received a SIGALRM */
 struct	ufs1_dinode ufs1_zino;
 struct	ufs2_dinode ufs2_zino;
 
+struct dups *duplist;
+struct dups *muldup;
+struct inostatlist *inostathead;
+
 void
 fsckinit(void)
 {

sbin/fsck_ffs/setup.c

@@ -58,6 +58,8 @@ __FBSDID("$FreeBSD$");
 
 #include "fsck.h"
 
+struct inoinfo **inphead, **inpsort;
+
 struct uufsd disk;
 struct bufarea asblk;
 #define altsblock (*asblk.b_un.b_fs)

sbin/fsdb/fsdb.c

@@ -70,9 +70,6 @@ usage(void)
 	exit(1);
 }
 
-int returntosingle;
-char nflag;
-
 /*
  * We suck in lots of fsck code, and just pick & choose the stuff we want.
  *