bcachefs: Run bch2_check_dirent_target() at lookup time

bcachefs: Run bch2_check_dirent_target() at lookup time

More on the "full online self healing" project:

We now run most of the dirent <-> inode consistency checks, with repair
code, at runtime - the exact same check and repair code that fsck runs.

This will allow us to repair the "dirent points to inode that does not
point back" inconsistencies that have been popping up at runtime.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>