Fix daily_backup_gpart_exclude in periodic daily 221.backup-gpart

Since gpart_devs was not quoted (losing embedded newlines), if
daily_backup_gpart_exclude matched something, gpart_devs was empty.

PR: 251961
Submitted by: Kan Sasaki

(cherry picked from commit 93900fc697992f893e2e69dce0c2f67b1f66a774)