[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 <viro@zeniv.linux.org.uk>

security/keys/keyctl.c

@@ -1074,7 +1074,7 @@ long keyctl_instantiate_key_common(key_serial_t id,
 		}
 
 		ret = -EFAULT;
-		if (copy_from_iter(payload, plen, from) != plen)
+		if (!copy_from_iter_full(payload, plen, from))
 			goto error2;
 	}