pvcalls-front: Avoid get_free_pages(GFP_KERNEL) under spinlock

The problem is that we call this with a spin lock held.
The call tree is:
pvcalls_front_accept() holds bedata->socket_lock.
    -> create_active()
        -> __get_free_pages() uses GFP_KERNEL

The create_active() function is only called from pvcalls_front_accept()
with a spin_lock held, The allocation is not allowed to sleep and
GFP_KERNEL is not sufficient.

This issue was detected by using the Coccinelle software.

v2: Add a function doing the allocations which is called
    outside the lock and passing the allocated data to
    create_active().

v3: Use the matching deallocators i.e., free_page()
    and free_pages(), respectively.

v4: It would be better to pre-populate map (struct sock_mapping),
    rather than introducing one more new struct.

v5: Since allocating the data outside of this call it should also
    be freed outside, when create_active() fails.
    Move kzalloc(sizeof(*map2), GFP_ATOMIC) outside spinlock and
    use GFP_KERNEL instead.

v6: Drop the superfluous calls.

Suggested-by: Juergen Gross <jgross@suse.com>
Suggested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Suggested-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
CC: Julia Lawall <julia.lawall@lip6.fr>
CC: Boris Ostrovsky <boris.ostrovsky@oracle.com>
CC: Juergen Gross <jgross@suse.com>
CC: Stefano Stabellini <sstabellini@kernel.org>
CC: xen-devel@lists.xenproject.org
CC: linux-kernel@vger.kernel.org
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>