fsck_ffs: fix background fsck in preen mode

Background checks are only allowed for mounted filesystems - don't try
to open the device for writing when performing a background check.

While here, remove a debugging printf that's commented out.

PR:             256746
Fixes:          5cc5263
Reviewed by:	mckusick
Differential Revision:	https://reviews.freebsd.org/D30880

(cherry picked from commit 0c5a592)

sbin/fsck_ffs/main.c

@@ -295,15 +295,14 @@ checkfilesys(char *filesys)
 		 */
 		if ((fsreadfd = open(filesys, O_RDONLY)) < 0 || readsb(0) == 0)
 			exit(3);	/* Cannot read superblock */
-		if (nflag || (fswritefd = open(filesys, O_WRONLY)) < 0) {
+		if (bkgrdflag == 0 &&
+		    (nflag || (fswritefd = open(filesys, O_WRONLY)) < 0)) {
 			fswritefd = -1;
 			if (preen)
 				pfatal("NO WRITE ACCESS");
 			printf(" (NO WRITE)");
 		}
 		if ((sblock.fs_flags & FS_GJOURNAL) != 0) {
-			//printf("GJournaled file system detected on %s.\n",
-			//    filesys);
 			if (sblock.fs_clean == 1) {
 				pwarn("FILE SYSTEM CLEAN; SKIPPING CHECKS\n");
 				exit(0);
@@ -317,10 +316,10 @@ checkfilesys(char *filesys)
 			} else {
 				pfatal(
 			    "UNEXPECTED INCONSISTENCY, CANNOT RUN FAST FSCK\n");
-				close(fsreadfd);
-				close(fswritefd);
 			}
 		}
+		close(fsreadfd);
+		close(fswritefd);
 	}
 	/*
 	 * If we are to do a background check: