xfs: inode-related repair fixes [v29.5 34/71]

While doing QA of the online fsck code, I made a few observations:
First, nobody was checking that the di_onlink field is actually zero;
Second, that allocating a temporary file for repairs can fail (and
thus bring down the entire fs) if the inode cluster is corrupt; and
Third, that file link counts do not pin at ~0U to prevent integer
overflows.  Fourth, the x{chk,rep}_metadata_inode_fork functions
should be subclassing the main scrub context, not modifying the
parent's setup willy-nilly.

This scattered patchset fixes those three problems.

This has been running on the djcloud for months with no problems.  Enjoy!

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