ceph: try getting buffer capability for readahead/fadvise

commit 2b1ac852eb67a6e95595e576371d23519105559f upstream.

For readahead/fadvise cases, caller of ceph_readpages does not
hold buffer capability. Pages can be added to page cache while
there is no buffer capability. This can cause data integrity
issue.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>