nfp: use kvcalloc() to allocate SW buffer descriptor arrays

Use kvcalloc() instead of tmp variable + kzalloc() when allocating
SW buffer information to allow falling back to vmalloc and to protect
from theoretical integer overflow.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>