[brown paperbag] fix coredump breakage

Let me count the ways in which I'd screwed up:

* when emitting a page, handling of gaps in coredump should happen
before fetching the current file position.
* fix for a problem that occurs on rather uncommon setups (and hadn't
been observed in the wild) had been sent very late in the cycle.
* ... with badly insufficient testing, introducing an easily
reproducible breakage.  Without giving it time to soak in -next.
Fucked-up-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: "J. R. Okajima" <hooanon05g@gmail.com>
Tested-by: "J. R. Okajima" <hooanon05g@gmail.com>
Fixes: 06bbaa6d "[coredump] don't use __kernel_write() on kmap_local_page()"
Cc: stable@kernel.org	# v6.0-only
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/coredump.c

@@ -841,7 +841,7 @@ static int dump_emit_page(struct coredump_params *cprm, struct page *page)
 	};
 	struct iov_iter iter;
 	struct file *file = cprm->file;
-	loff_t pos = file->f_pos;
+	loff_t pos;
 	ssize_t n;
 
 	if (cprm->to_skip) {
@@ -853,6 +853,7 @@ static int dump_emit_page(struct coredump_params *cprm, struct page *page)
 		return 0;
 	if (dump_interrupted())
 		return 0;
+	pos = file->f_pos;
 	iov_iter_bvec(&iter, WRITE, &bvec, 1, PAGE_SIZE);
 	n = __kernel_write_iter(cprm->file, &iter, &pos);
 	if (n != PAGE_SIZE)