client: LAZY_IO support

client: LAZY_IO support

'client_force_lazyio' option enables LAZY_IO gloablly for libcephfs
and ceph-fuse mount.

'ceph_lazyio(...)' and 'ceph_ll_lazyio(...)' enable LAZY_IO for open
file in libcephfs.

'ioctl(fd, CEPH_IOC_LAZYIO, 1UL)' enables LAZY_IO for open fils in
ceph-fuse mount.

Fixes: http://tracker.ceph.com/issues/20598
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>