Fix several unsafe pointer dereferences in the buffered_write() function, implementing the sysctl vfs.ffs.set_bufoutput (not used in the tree yet).
Fix several unsafe pointer dereferences in the buffered_write()
function, implementing the sysctl vfs.ffs.set_bufoutput (not used in
the tree yet).

- The current directory vnode dereference is unsafe since fd_cdir
  could be changed and unreferenced, lock the filedesc around and vref
  the fd_cdir.
- The VTOI() conversion of the fd_cdir is unsafe without first
  checking that the vnode is indeed from an FFS mount, otherwise
  the code dereferences a random memory.
- The cdir could be reclaimed from under us, lock it around the
  checks.
- The type of the fp vnode might be not a disk, or it might have
  changed while the thread was in flight, check the type.

Reviewed and tested by:	mckusick
MFC after:	2 weeks

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase