xsk: Fix clang build error in __xp_alloc

Fix a build error with clang in __xp_alloc():

  [...]
  net/xdp/xsk_buff_pool.c:465:15: error: variable 'xskb' is uninitialized
  when used here [-Werror,-Wuninitialized]
                          xp_release(xskb);
                                     ^~~~

This is correctly detected by clang, but not gcc. In fact, the xp_release()
statement should not be there at all in the refactored code, just remove it.

Fixes: 94033cd8 ("xsk: Optimize for aligned case")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20210929061403.8587-1-magnus.karlsson@gmail.com