XArray updates for 5.0-rc3

Fix some oversights in the XArray porcelain API:
 - support for m68k's two-byte aligned pointers
 - reserving entries using xa_insert()
 - missing xa_insert_bh() and xa_insert_irq() functions
 - simplify using xa_for_each()
 - use lockdep correctly
 - a few other minor fixes and improvements
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