• Make 'struct buf *buf' private to vfs_bio.c. Having a global variable 'buf' is inconvenient and has lead me to some irritating to discover bugs over the years. It also makes it more challenging to refactor the buf allocation system.
• Move swbuf and declare it as an extern in vfs_bio.c. This is still not perfect but better than it was before.
• Eliminate the unused ffs function that relied on knowledge of the buf array.
• Move the shutdown code that iterates over the buf array into vfs_bio.c.

Reviewed by: kib
Sponsored by: EMC / Isilon Storage Division