misc: ibmvmc: Use GFP_ATOMIC under spin lock

The function alloc_dma_buffer() is called from ibmvmc_add_buffer(),
in which a spin lock be held here, so we should use GFP_ATOMIC when
a lock is held.

Fixes: 0eca353e

 ("misc: IBM Virtual Management Channel Driver (VMC)")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Bryant G. Ly <bryantly@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>