Ubuntu 22.04 integration: ShellCheck

- Add new SC2312 global exclude.
  ```
  Consider invoking this command separately to avoid masking its return
  value (or use '|| true' to ignore). [SC2312]
  ```

- Correct errors detected by new ShellCheck version.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #14148

autogen.sh

@@ -16,7 +16,7 @@ automake --version | awk '{print $NF; exit}' | (
 			dir="${dir%/Makefile.am}"
 			grep -q '%[CD]%' "$dir/Makefile.am" || continue
 
-			reldir="${dir#$root}"
+			reldir="${dir#"$root"}"
 			reldir="${reldir#/}"
 
 			canon_reldir="$(printf '%s' "$reldir" | tr -C 'a-zA-Z0-9@_' '_')"