gfs2: Fix underflow in gfs2_page_mkwrite

On filesystems with a block size smaller than PAGE_SIZE and non-empty
files smaller then PAGE_SIZE, gfs2_page_mkwrite could end up allocating
excess blocks beyond the end of the file, similar to fallocate.  This
doesn't make sense; fix it.

Reported-by: Bob Peterson <rpeterso@redhat.com>
Fixes: 184b4e6 ("gfs2: Fix end-of-file handling in gfs2_page_mkwrite")
Cc: stable@vger.kernel.org # v5.5+
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

Author: Andreas Gruenbacher

fs/gfs2/file.c

@@ -450,8 +450,8 @@ static vm_fault_t gfs2_page_mkwrite(struct vm_fault *vmf)
 	file_update_time(vmf->vma->vm_file);
 
 	/* page is wholly or partially inside EOF */
-	if (offset > size - PAGE_SIZE)
-		length = offset_in_page(size);
+	if (size - offset < PAGE_SIZE)
+		length = size - offset;
 	else
 		length = PAGE_SIZE;