xfs: rewrite xfs_icache_inode_is_allocated

Back in the mists of time[1], I proposed this function to assist the
inode btree scrubbers in checking the inode btree contents against the
allocation state of the inode records.  The original version performed a
direct lookup in the inode cache and returned the allocation status if
the cached inode hadn't been reused and wasn't in an intermediate state.
Brian thought it would be better to use the usual iget/irele mechanisms,
so that was changed for the final version.

Unfortunately, this hasn't aged well -- the IGET_INCORE flag only has
one user and clutters up the regular iget path, which makes it hard to
reason about how it actually works.  Worse yet, the inode inactivation
series silently broke it because iget won't return inodes that are
anywhere in the inactivation machinery, even though the caller is
already required to prevent inode allocation and freeing.  Inodes in the
inactivation machinery are still allocated, but the current code's
interactions with the iget code prevent us from being able to say that.

Now that I understand the inode lifecycle better than I did in early
2017, I now realize that as long as the cached inode hasn't been reused
and isn't actively being reclaimed, it's safe to access the i_mode field
(with the AGI, rcu, and i_flags locks held), and we don't need to worry
about the inode being freed out from under us.

Therefore, port the original version to modern code structure, which
fixes the brokennes w.r.t. inactivation.  In the next patch we'll remove
IGET_INCORE since it's no longer necessary.

[1] https://lore.kernel.org/linux-xfs/149643868294.23065.8094890990886436794.stgit@birch.djwong.org/

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

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