xsk: fix batch alloc API on non-coherent systems

xsk: fix batch alloc API on non-coherent systems

In cases when synchronizing DMA operations is necessary,
xsk_buff_alloc_batch() returns a single buffer instead of the requested
count. This puts the pressure on drivers that use batch API as they have
to check for this corner case on their side and take care of allocations
by themselves, which feels counter productive. Let us improve the core
by looping over xp_alloc() @max times when slow path needs to be taken.

Another issue with current interface, as spotted and fixed by Dries, was
that when driver called xsk_buff_alloc_batch() with @max == 0, for slow
path case it still allocated and returned a single buffer, which should
not happen. By introducing the logic from first paragraph we kill two
birds with one stone and address this problem as well.

Fixes: 47e4075df300 ("xsk: Batched buffer allocation for the pool")
Reported-and-tested-by: Dries De Winter <ddewinter@synamedia.com>
Co-developed-by: Dries De Winter <ddewinter@synamedia.com>
Signed-off-by: Dries De Winter <ddewinter@synamedia.com>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20240911191019.296480-1-maciej.fijalkowski@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>