o Remove the initialization of unused fields in the struct

o Remove the initialization of unused fields in the struct
  uio now that we don't use uiomove() anymore.
o Enforce stricter checks on the length of the iov's in
  nmount(2) since we now malloc() them individually and
  corrupted iov's could make the kernel crash in malloc()
  with "kmem_map too small".

Reviewed by:	phk