pgsql: Fix using too many LWLocks bug, reported by Craig Ringer

Log Message:
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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.

Tags:
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REL8_2_STABLE

Modified Files:
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    pgsql/src/backend/access/gin:
        ginget.c (r1.4.2.1 -> r1.4.2.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginget.c?r1=1.4.2.1&r2=1.4.2.2)
    pgsql/src/include/access:
        gin.h (r1.9.2.1 -> r1.9.2.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/gin.h?r1=1.9.2.1&r2=1.9.2.2)