afs: Fix page overput in afs_fill_page()

afs_fill_page() loads the page it wants to fill into the afs_read request
without incrementing its refcount - but then calls afs_put_read() to clean
up afterwards, which then releases a ref on the page.

Fix this by getting a ref on the page before calling
afs_vnode_fetch_data().

This causes sync after a write to hang in afs_writepages_region() because
find_get_pages_tag() gets confused and doesn't return.

Fixes: 196ee9cd ("afs: Make afs_fs_fetch_data() take a list of pages")
Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Marc Dionne <marc.dionne@auristor.com>

fs/afs/write.c

@@ -101,6 +101,7 @@ static int afs_fill_page(struct afs_vnode *vnode, struct key *key,
 	req->pos = pos;
 	req->nr_pages = 1;
 	req->pages[0] = page;
+	get_page(page);
 
 	i_size = i_size_read(&vnode->vfs_inode);
 	if (pos + PAGE_SIZE > i_size)