sfc: Add GFP flags to efx_nic_alloc_buffer() and make most callers allow blocking

Most call sites for efx_nic_alloc_buffer() are part of the probe or
reconfiguration paths and can allocate with GFP_KERNEL.  A few others
should use GFP_NOIO (I think).  Only one is in atomic context and
must use the current GFP_ATOMIC.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>