Unlock correct lock in ffs_snapblkfree().

Unlock correct lock in ffs_snapblkfree().

It is possible for ffs_snapblkfree() to race and lock snaplock while
the devvp snapdata is instantiated, but no snapshots exist.  In this
case the loop over snapshots in ffs_snapblkfree() is not executed, and
the local variable vp is left initialized to NULL.

Unlock using &sn->sn_lock and not vp->v_vnlock.  For the inodes on the
snapshot list, the locks are same.

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