vmcore: convert read_from_oldmem() to take an iov_iter

Remove the read_from_oldmem() wrapper introduced earlier and convert all
the remaining callers to pass an iov_iter.

Link: https://lkml.kernel.org/r/20220408090636.560886-4-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>
Cc: Tiezhu Yang <yangtiezhu@loongson.cn>
Cc: Amit Daniel Kachhap <amit.kachhap@arm.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>