ceph: allocate sparse_ext map only for sparse reads

If mounted with sparseread option, ceph_direct_read_write() ends up
making an unnecessarily allocation for O_DIRECT writes.

Fixes: 03bc06c7 ("ceph: add new mount option to enable sparse reads")
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Alex Markuze <amarkuze@redhat.com>