[iov_iter] new primitives - copy_from_iter_full() and friends

copy_from_iter_full(), copy_from_iter_full_nocache() and
csum_and_copy_from_iter_full() - counterparts of copy_from_iter()
et.al., advancing iterator only in case of successful full copy
and returning whether it had been successful or not.

Convert some obvious users.  *NOTE* - do not blindly assume that
something is a good candidate for those unless you are sure that
not advancing iov_iter in failure case is the right thing in
this case.  Anything that does short read/short write kind of
stuff (or is in a loop, etc.) is unlikely to be a good one.

Signed-off-by: Al Viro <[email protected]>

fs/ncpfs/file.c

@@ -203,7 +203,7 @@ ncp_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 				      bufsize - (pos % bufsize),
 				      iov_iter_count(from));
 
-		if (copy_from_iter(bouncebuffer, to_write, from) != to_write) {
+		if (!copy_from_iter_full(bouncebuffer, to_write, from)) {
 			errno = -EFAULT;
 			break;
 		}