client/fuse: handle case of renameat2 with non-zero flags

client/fuse: handle case of renameat2 with non-zero flags

When user issues renameat(2) with non-zero flags (RENAME_EXCHANGE or
RENAME_NOREPALCE) the current code ignores those flags and treat the
call as ordinary rename. This, in turn, may yield successful rename with
wrong semantics then those expected by the caller.

Follow the same semantics as kernel's cephfs client: return -EINVAL when
having non-zero flags to renameat2 (see 'ceph_rename' at fs/ceph/dir.c).

Fixes: https://tracker.ceph.com/issues/63722
Signed-off-by: Shachar Sharon <ssharon@redhat.com>