Fix using too many LWLocks bug, reported by Craig Ringer

Fix using too many LWLocks bug, reported by Craig Ringer
<craig@postnewspapers.com.au>.
It was my mistake, I missed limitation of number of held locks, now GIN doesn't
use continiuous locks, but still hold buffers pinned to prevent interference
with vacuum's deletion algorithm.

Backpatch is needed.