Improve publication of the newly allocated snapdata.

For freshly allocated snapdata, Lock sn_lock in advance, so
si_snapdata readers see the locked snapdata and not race.

For existing snapdata, if the thread was put to sleep waiting for
sn_lock, re-read si_snapdata. This either closes the race or makes
the reliance on LK_DRAIN less important.

Reported and tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks