new helper: dup_iter()

Copy iter and kmemdup the underlying array for the copy.  Returns
a pointer to result of kmemdup() to be kfree()'d later.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc include/linux/uio.h
Simple merge

diff --cc mm/iov_iter.c
Simple merge