This speeds up vsm_findseg by keeping a direct pointer to the struct
vsm_seg it points to in the vsm_fantom.

To prevent stale vsm_fantoms in an application from causing segfaults, the
last serial number seen is also kept on the fantom. If this serial number
does not match, the slow path of iterating all segs to find the right
match is done, and the fantom is updated to make any subsequent calls on
the same fantom take the fast path.

With this patch the fantoms store 2 serial numbers and a direct pointer to
the seg. To fit this in struct vsm_fantom without breaking the ABI, the
unused chunk pointer value has been repurposed, giving us two uintptr_t
priv variables to work with. The direct segment pointer occupies one, and
the two serial numbers occupy one half each of the other. This limits the
bits for each serial to only 16 bits on 32 bit systems. But only direct
matches is done, and it is unlikely that a stale fantom should have the
exact right value to be accepted as valid.