LU-16313 pcc: use two bits to indicate pcc type for attach

LU-16313 pcc: use two bits to indicate pcc type for attach

PCC currenty supports two types: readwrite and readonly.
The attach data structure @lu_pcc_attach is using 32 bit value to
indicate the PCC type:
	struct lu_pcc_attach {
		__u32 pcca_type;
		__u32 pcca_id;
	};

In this patch, it changes to use 2 bits to represent the PCC type.
The left bits in @pcca_type can be used as flags for attach such
as a flag to indicate using the asynchronous attach via the
command "lfs pcc attach -A" for PCCRO.

Lustre-change: https://review.whamcloud.com/49160
Lustre-commit: 6e90974b1f4ac24c5a5d45ecc9bdb4d47018dab4

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: Idee26018642a174b04d1d36a81952ea98a06514e
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/49163
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>