Pass --enable=all to shellcheck within contrib/

- Remove `SHELLCHECK_IGNORE` in favor of inline suppressions
  and more general `SHELLCHECK_OPTS`.

- Exclude `SC2250` (turned on by `--enable=all`) globally

- Pass `--enable=all` to shellcheck for scripts in contrib/: it's
  very important to catch errors early in areas that are not easily
  testable.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #12760

contrib/initramfs/scripts/Makefile.am

@@ -9,3 +9,4 @@ SUBDIRS = local-top
 
 SHELLCHECKDIRS = $(SUBDIRS)
 SHELLCHECK_SHELL = sh
+SHELLCHECK_OPTS = --enable=all

contrib/initramfs/scripts/local-top/Makefile.am

@@ -4,3 +4,5 @@ localtopdir = /usr/share/initramfs-tools/scripts/local-top
 
 dist_localtop_SCRIPTS = \
         zfs
+
+SHELLCHECK_OPTS = --enable=all

contrib/initramfs/scripts/zfs

@@ -596,7 +596,7 @@ setup_snapshot_booting()
 	retval=0
 
 	# Make sure that the snapshot specified actually exists.
-	if [ ! "$(get_fs_value "${snap}" type)" ]
+	if [ -z "$(get_fs_value "${snap}" type)" ]
 	then
 		# Snapshot does not exist (...@<null> ?)
 		# ask the user for a snapshot to use.
@@ -613,7 +613,8 @@ setup_snapshot_booting()
 	then
 		# If the destination dataset for the clone
 		# already exists, destroy it. Recursively
-		if [ "$(get_fs_value "${rootfs}_${snapname}" type)" ]; then
+		if [ -n "$(get_fs_value "${rootfs}_${snapname}" type)" ]
+		then
 			filesystems=$("${ZFS}" list -oname -tfilesystem -H \
 			    -r -Sname "${ZFS_BOOTFS}")
 			for fs in $filesystems; do
@@ -848,7 +849,7 @@ mountroot()
 		done
 		IFS="$OLD_IFS"
 
-		[ "$quiet" != "y" ] && zfs_log_end_msg $ZFS_ERROR
+		[ "$quiet" != "y" ] && zfs_log_end_msg "$ZFS_ERROR"
 	else
 		# No auto - use value from the command line option.