osd: disable omap if the pool is not replicated

If one of the operations in MOSDOp is CEPH_OSD_OP_OMAP* and the pool is
not replicated, return EINVAL.

When an erasure coded pool is created set the NO_MAP flag and use it to
disable all omap operations.

http://tracker.ceph.com/issues/6895 fixes #6895

Signed-off-by: Loic Dachary <loic@dachary.org>
osd: add osd pool set/get no_omap

so omap support can be set on a per pool basis (unless it is erasure
coded in which case it is always disabled).

Signed-off-by: Loic Dachary <loic@dachary.org>