libcephfs: add ceph_ll_lookup_root()

Signed-off-by: Andrey Kuznetsov <Andrey_Kuznetsov@epam.com>
libcephfs: make readlink copy data to users buffer

Previous we were copying a private poiner to the caller.

Signed-off-by: Andrey Kuznetsov <Andrey_Kuznetsov@epam.com>
libcephfs: add ceph_ll_listxattr

Signed-off-by: Andrey Kuznetsov <Andrey_Kuznetsov@epam.com>
libcephfs: add an xattr ll test

Signed-off-by: Andrey Kuznetsov <Andrey_Kuznetsov@epam.com>
mds: include requested caps (namely, xattrs) on getattr reply

For xattrs, we only include them in the reply if we are issuing caps on
them.  However, in the getattr case, we need to include the snapshot of the
current state.

The original problem is reproduced by LibCephFS.Xattrs_ll (a simple set
then get).

Signed-off-by: Sage Weil <sage@inktank.com>