[$] Alignment guarantees for kmalloc()

kmalloc() is one of the kernel's fundamental memory-allocation
primitives for relatively small objects. Most of the time, developers
don't worry about the alignment of memory returned from
kmalloc(), and things generally just work. But, Vlastimil Babka
said during a plenary session at the 2019 Linux Storage,
Filesystem, and Memory-Management Summit, every now and then
kmalloc() will do something surprising. He proposed tightening
the guarantees around object alignment in the hope of generating fewer
surprises in the future.