fsck.zfs: implement 4/8 exit codes as suggested in manpage

Update the fsck.zfs helper to bubble up some already-known-about 
errors if they are detected in the pool.

health=degraded => 4/"Filesystem errors left uncorrected"
health=faulted && dataset in /etc/fstab => 8/"Operational error"
pool not found => 8/"Operational error"
everything else => 0

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11806

cmd/fsck_zfs/.gitignore

@@ -0,0 +1 @@
+/fsck.zfs

cmd/fsck_zfs/Makefile.am

@@ -1 +1,5 @@
+include $(top_srcdir)/config/Substfiles.am
+
 dist_sbin_SCRIPTS = fsck.zfs
+
+SUBSTFILES += $(dist_sbin_SCRIPTS)

cmd/fsck_zfs/fsck.zfs

@@ -1,9 +0,0 @@
-#!/bin/sh
-#
-# fsck.zfs: A fsck helper to accommodate distributions that expect
-# to be able to execute a fsck on all filesystem types.  Currently
-# this script does nothing but it could be extended to act as a
-# compatibility wrapper for 'zpool scrub'.
-#
-
-exit 0

cmd/fsck_zfs/fsck.zfs.in

@@ -0,0 +1,44 @@
+#!/bin/sh
+#
+# fsck.zfs: A fsck helper to accommodate distributions that expect
+# to be able to execute a fsck on all filesystem types.
+#
+# This script simply bubbles up some already-known-about errors,
+# see fsck.zfs(8)
+#
+
+if [ "$#" = "0" ]; then
+	echo "Usage: $0 [options] dataset…" >&2
+	exit 16
+fi
+
+ret=0
+for dataset in "$@"; do
+	case "$dataset" in
+		-*)
+			continue
+			;;
+		*)
+			;;
+	esac
+
+	pool="${dataset%%/*}"
+
+	case "$(@sbindir@/zpool list -Ho health "$pool")" in
+		DEGRADED)
+			ret=$(( $ret | 4 ))
+			;;
+		FAULTED)
+			awk '!/^([[:space:]]*#.*)?$/ && $1 == "'"$dataset"'" && $3 == "zfs" {exit 1}' /etc/fstab || \
+				ret=$(( $ret | 8 ))
+			;;
+		"")
+			# Pool not found, error printed by zpool(8)
+			ret=$(( $ret | 8 ))
+			;;
+		*)
+			;;
+	esac
+done
+
+exit "$ret"