mm/vmscan: print err before panic

If panic is enable,the err information will not be printed before bugon,
So swap it.  Print the return value of PTR_ERR(pgdat->kswapd) also.

Link: https://lkml.kernel.org/r/20230906083700.181-1-angus.chen@jaguarmicro.com


Signed-off-by: Angus Chen <angus.chen@jaguarmicro.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>