swiotlb: allocate a new memory pool when existing pools are full

When swiotlb_find_slots() cannot find suitable slots, schedule the
allocation of a new memory pool. It is not possible to allocate the pool
immediately, because this code may run in interrupt context, which is not
suitable for large memory allocations. This means that the memory pool will
be available too late for the currently requested mapping, but the stress
on the software IO TLB allocator is likely to continue, and subsequent
allocations will benefit from the additional pool eventually.

Keep all memory pools for an allocator in an RCU list to avoid locking on
the read side. For modifications, add a new spinlock to struct io_tlb_mem.

The spinlock also protects updates to the total number of slabs (nslabs in
struct io_tlb_mem), but not reads of the value. Readers may therefore
encounter a stale value, but this is not an issue:

- swiotlb_tbl_map_single() and is_swiotlb_active() only check for non-zero
  value. This is ensured by the existence of the default memory pool,
  allocated at boot.

- The exact value is used only for non-critical purposes (debugfs, kernel
  messages).

Signed-off-by: Petr Tesarik <petr.tesarik.ext@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>