[$] Block-layer bounce buffering bounces out of the kernel

As the end of the 1990s approached, a lot of kernel-development effort wasgoing into improving support for 32-bit systemswith shockingly large amounts of memory installed. This being the 1990s,having more than 1GB of memory in such a system was deemed to be shocking.Many of the compromises made to support such inconceivably large systemshave remained in the kernel to this day. One of those compromises -bounce buffering of I/O requests in the block layer - has finally beeneased out for the 6.16 release, more than a quarter-century after itsintroduction.