Revert "Use vmalloc instead of kmalloc for request buffers."

This reverts commit 4dad095ede1967a08324253ab6c5f7e1bcc4a838.

Although this worked for me when I originally tested it (made
fileystems, etc.)  It did not work for Chayan on 3.5 kernel, for
example, or RHEL6 kernels.  So, let's revert it.