xfs: fix iget usage in directory scrub

In this series, we fix some problems with how the directory scrubber
grabs child inodes.  First, we want to reduce EDEADLOCK returns by
replacing fixed-iteration loops with interruptible trylock loops.
Second, we add UNTRUSTED to the child iget call so that we can detect a
dirent that points to an unallocated inode.  Third, we fix a bug where
we weren't checking the inode pointed to by dotdot entries at all.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
-----BEGIN PGP SIGNATURE-----

iHUEABYKAB0WIQQ2qTKExjcn+O1o2YRKO3ySh0YRpgUCY8c+BwAKCRBKO3ySh0YR
pnkdAQCvnugApuzaSsVSslbRWnLCswAXeByJWsaI8hhftT3CdAD/TBHN14HZfaCu
pxVGBJlpCCjqYO30Lsa4AgVm7fWcgwM=
=TjKN
-----END PGP SIGNATURE-----