proc/vmcore: fix signedness bug in read_from_oldmem()

The bug is the error handling:

if (tmp < nr_bytes) {

"tmp" can hold negative error codes but because "nr_bytes" is type size_t
the negative error codes are treated as very high positive values
(success). Fix this by changing "nr_bytes" to type ssize_t. The
"nr_bytes" variable is used to store values between 1 and PAGE_SIZE and
they can fit in ssize_t without any issue.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 5d8de293c224 ("vmcore: convert copy_oldmem_page() to take an iov_iter")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Matthew Wilcox (Oracle) <[e-mail]>
Acked-by: Baoquan He <[e-mail]>
Cc: Dave Young <[e-mail]>
Cc: Vivek Goyal <[e-mail]>
Cc: Alexey Dobriyan <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

fs/proc/vmcore.c

···
 			 u64 *ppos, bool encrypted)
 {
 	unsigned long pfn, offset;
-	size_t nr_bytes;
+	ssize_t nr_bytes;
 	ssize_t read = 0, tmp;
 	int idx;