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: 5cc52631b3b88dfc36d8049dc8bece8573c5f9af
Reviewed by: mckusick
Differential Revision: https://reviews.freebsd.org/D30880

(cherry picked from commit 0c5a59252c8e7b80b98521ebc23a415a05ff9594)