nfp: bpf: round up the size of the stack

Kernel enforces the alignment of the bottom of the stack, NFP
deals with positive offsets better so we should align the top
of the stack.  Round the stack size to NFP word size (4B).

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>