Add an iterator data structure to operate over an iovec.  Add usercopy
operators needed by generic_file_buffered_write, and convert that function
over.
Signed-off-by: NNick Piggin <npiggin@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>