xsk: Move defines only used by AF_XDP internals to xsk.h

Move the XSK_NEXT_PG_CONTIG_{MASK,SHIFT}, and
XDP_UMEM_USES_NEED_WAKEUP defines from xdp_sock.h to the AF_XDP
internal xsk.h file. Also, start using the BIT{,_ULL} macro instead of
explicit shifts.

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200520192103.355233-5-bjorn.topel@gmail.com