cifs_iovec_read: keep iov_iter between the calls of cifs_readdata_to_iov()

... we are doing them on adjacent parts of file, so what happens is that
each subsequent call works to rebuild the iov_iter to exact state it
had been abandoned in by previous one.  Just keep it through the entire
cifs_iovec_read().  And use copy_page_to_iter() instead of doing
kmap/copy_to_user/kunmap manually...

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