(Patch has been ACKed by the bfs maintainer)

Now that bfs no more is included in the big unions in fs.h it makes
sense to move the contents of bfs_fs_i.h and bfs_fs_sb.h to a bfs-private
location.  I've created fs/bfs/bfs.h for that, also merging in bfs_defs.h.

In addition I've changed si_imap to an unsigned long pointer as the bitops
now use that type explicitly.