constify iov_iter_count() and iter_is_iovec()

commit b57332b4 upstream.

[stable note, need this to prevent build warning in commit
a0ac402c]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>