LU-10499 pcc: add readonly mode for PCC

LU-10499 pcc: add readonly mode for PCC

Readonly Persistent Client Cache (RO-PCC) shares the same framework
with Readwrite Persistent Client Cache, expect that no HSM mechanism
is used in readonly mode of PCC. Instead, RO-PCC adds a new flag
field in the file object's layout named LOV_PATTERN_F_RDONLY to
indicate that the file is in PCC read-only state. It is protected
under the layout lock.

After introducing the readonly feature for the layout, the IO path
has some changes. For read, if the file has been valid RO-PCC
cached, the file data can be read from PCC directly; Otherwise, it
will read data using normal I/O path from OSTs. For data modifying
operations (write or truncate), it must clear the readonly flag of
the layout on MDT (which will invaliate the RO-PCC cached state on
clients via layout lock blocking callback), and then it can perform
I/O.

For RO-PCC, as the PCC cached file is actual a replication of
Lustre file, when data read on PCC failed, it can tolerate this
error by falling back to normal read path: read data from OSTs.

This patch also combines PCC-RO with FLR. Similar to the plain
layouts, PCC-RO layouts is a kind of HSM non-composite layouts,
can be treated as a basic mirror component in FLR layouts.

Test-Parameters: clientcount=3 testlist=sanity-pcc,sanity-pcc,sanity-pcc
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I6badd72e00a106a0f68950621ce6f82471731a95