xfs: use slab caches for deferred log items

Since the adding of reflink and reverse mapping to the filesystem,
deferred log items are used quite a lot more frequently than they used
to be.  This means that we're cycling a lot of small objects through the
slab caches, just like we do with btree cursors.  Pack deferred item
contexts into memory pages more densely by creating separate slab caches
for each type, and shrink the intent items to use less memory.
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