xfs_repair: don't crash on unknown inode parents in dry run mode

xfs_repair: don't crash on unknown inode parents in dry run mode

Fuzz testing of directory block headers exposed a debug assertion vector
in xfs_repair.  In normal (aka fixit) mode, if a single-block directory
has a totally trashed block, repair will zap the entire directory.
Phase 4 ignores any dirents pointing to the zapped directory, phase 6
ignores the freed directory, and everything is good.

However, in dry run mode, we don't actually free the inode.  Phase 4
still ignores any dirents pointing to the zapped directory, but phase 6
thinks the inode is still live and tries to walk it.  xfs_repair doesn't
know of any parents for the zapped directory and so trips the assertion.

The assertion is critical for fixit mode because we need all the parent
information to ensure consistency of the directory tree.  In dry run
mode we don't care, because we only have to print inconsistencies and
return 1.  Worse yet, (our) customers file bugs when xfs_repair crashes
during a -n scan, so this will generate support calls.

Make everyone's life easier by downgrading the assertion to a warning if
we're running in dry run mode.

Found by fuzzing bhdr.hdr.bno = zeroes in xfs/471.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>