[PATCH] pci/pool.c less spinlock abuse

[PATCH] pci/pool.c less spinlock abuse

That previous patch got rid of a boot time might_sleep() warning,
but I noticed two later on:

  - kmalloc() needed SLAB_ATOMIC

  - destroying the 'pools' driverfs attribute could sleep too

The clean/simple patch for the second one tweaked an API:

  - pci_pool_create() can't be called in_interrupt() any more.
    nobody used it there, and such support isn't needed; plus
    that rule matches its sibling call, pci_pool_destroy().

  - that made its SLAB_* flags parameter more useless, so it's
    removed and the DMA-mapping.txt is updated.  (this param
    was more trouble than it was worth -- good that it's gone.)

Nobody (even DaveM) objected to those API changes, so I think
this should be merged.