s390/qeth: allocate initial TX Buffer structs with GFP_KERNEL

qeth_init_qdio_out_buf() is typically called during initialization, and
the GFP_ATOMIC is only needed for a very specific & rare case during TX
completion.

Allow callers to specify a gfp mask, so that the initialization path can
select GFP_KERNEL. While at it also clarify the function name.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>