zsmalloc: micro-optimize zs_object_copy()

zsmalloc: micro-optimize zs_object_copy()

A micro-optimization.  Avoid additional branching and reduce (a bit)
registry pressure (f.e.  s_off += size; d_off += size; may be calculated
twise: first for >= PAGE_SIZE check and later for offset update in "else"
clause).

scripts/bloat-o-meter shows some improvement

add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-10 (-10)
function                          old     new   delta
zs_object_copy                    550     540     -10

Signed-off-by: Sergey Senozhatsky <[email protected]>
Acked-by: Minchan Kim <[email protected]>
Cc: Nitin Gupta <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>