generic swap(): iphase: rename swap() to swap_byte_order()

In preparation for the introduction of a generic swap() macro.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>