LU-13602 pcc: add LCM_FL_PCC_RDONLY layout flag

The upcoming new feature PCC-RO is combined with FLR and extend
the on-disk data strucutre 'enum lov_comp_md_flags' for layout
components. It adds a new layout flag: LCM_FL_PCC_RDONLY.

enum lov_comp_md_flags {
	LCM_FL_NONE		= 0x0,
	LCM_FL_RDONLY		= 0x1,
	LCM_FL_WRITE_PENDING	= 0x2,
	LCM_FL_SYNC_PENDING	= 0x3,
	LCM_FL_PCC_RDONLY	= 0x8,
	LCM_FL_FLR_MASK         = 0xB,
};

The LCM_FL_PCC_RDONLY flag, which is dedicated for PCC-RO, is
different from LCM_FL_RDONLY.
A PCC-RO cached file could be in the state:
- LCM_FL_PCC_RDONLY | LCM_FL_RDONLY: it means that all FLR
  components are synced and in up-to-date state. The replicated
  file is on read-only state. And then one client attaches the
  file into the PCC backend with PCC-RO mode.
- LCM_FL_PCC_RDONLY | LCM_FL_WRITE_PENDING: it means the file was
  once modified, the data content of layout components are not
  synced. MDT has already picked a promary replica and marked
  other components as STALE. At this time, a client can still
  PCC-RO attach the file. On this client, the primary component
  and the PCC copy are both in up-to-date state.

As a new LCM_FL_PCC_RDONLY flag is added, the old client may not
understand this new FLR layout flag, and may result in
inconsistent data access.

This patch adds this new flag for the purpose of compatibility and
interoperability.

Test-Parameters: trivial
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I2e96f413c0b35355503c20dfea0a39d39a216d90
Reviewed-on: https://review.whamcloud.com/40813
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>