LU-17866 pcc: zero ra_pages explictly for a file after PCC mmap

LU-17866 pcc: zero ra_pages explictly for a file after PCC mmap

To support mmap under PCC, we do some special magic with mmap to
allow Lustre and PCC to share the page mapping.
The mapping host (@mapping->host) for the Lustre file is replaced
with the PCC copy for mmap. This may result in the wrong setting
of @ra_pages for the Lustre file handle with the backing store of
the PCC copy in the kernel:
->do_dentry_open()->file_ra_state_init():
file_ra_state_init(struct file_ra_state *ra,
		   struct address_space *mapping)
{
	ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
	ra->prev_pos = -1;
}

Setting readahead pages for a file handle is the last step of the
open() call and it is not under the control inside the Lustre file
system.
Thus, to avoid setting @ra_pages wrongly we set @ra_pages with
zero for Lustre file handle explictly in all read I/O path.

When invalidate a PCC copy, we will switch back the mapping
between Lustre and PCC. We also set mapping->a_ops back with
@ll_aops.
The readahead path in PCC backend may enter the ->readpage() in
Lustre. Then we check whethter the file handle is a Lustre file
handle. If not, it should be from mmap readahead I/O path of the
PCC copy and return error code directly in this case.

Change-Id: Id1e4a9e47bb484e97053759e1743fd2fce040149
Test-Parameters: clientdistro=el8.9 testlist=sanity-pcc env=ONLY=97,ONLY_REPEAT=10
Test-Parameters: clientdistro=el9.3 testlist=sanity-pcc env=ONLY=98,ONLY_REPEAT=10
Signed-off-by: Qian Yingjin <qian@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/55181
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>