LU-12811 ptlrpc: do some cleanups in swabber_dumper_helper()

LU-12811 ptlrpc: do some cleanups in swabber_dumper_helper()

This patch combines the process of dump/swab/dump calls for both
array and non-array cases in function swabber_dumper_helper(),
also does some code cleanups.

Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Change-Id: Iccd487999ad998068b4fae6a614bad3b17737d07
Reviewed-on: https://review.whamcloud.com/36848
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>