There are two structures in which a count of snapshots are
maintained:

    struct ceph_snap_context {
	...
        u32 num_snaps;
	...
    }
and
    struct ceph_snap_realm {
	...
        u32 num_prior_parent_snaps;   /*  had prior to parent_since */
	...
        u32 num_snaps;
	...
    }

These fields never take on negative values (e.g., to hold special
meaning), and so are really inherently unsigned.  Furthermore they
take their value from over-the-wire or on-disk formatted 32-bit
values.

So change their definition to have type u32, and change some spots
elsewhere in the code to account for this change.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>