EX-6468 pcc: add threshold to determine direct I/O during attach

EX-6468 pcc: add threshold to determine direct I/O during attach

This patch adds the threshold tunable parameter to determine doing
direct I/O or buffered I/O for data copying during attach:
	llite.*.pcc_dio_attach_threshold
The default value is same as direct I/O size: 32MiB.

And the usage of the parameter "pcc_dio_attach_size_mb" is
deprecated, and use "pcc_dio_attach_iosize_mb" instead.

Change-Id: I393d6a06523303e749192ba9978449c3d75886ae
Signed-off-by: Qian Yingjin <qian@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/49286
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Feng Lei <flei@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>