vmcore: convert __read_vmcore to use an iov_iter

This gets rid of copy_to() and let us use proc_read_iter() instead of
proc_read().

Link: https://lkml.kernel.org/r/20220408090636.560886-3-bhe@redhat.com


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>