pgsql: Fix oversight in coding of _bt_start_vacuum: we can't assume that

Log Message:
-----------
Fix oversight in coding of _bt_start_vacuum: we can't assume that the LWLock
will be released by transaction abort before _bt_end_vacuum gets called.
If either of these "can't happen" errors actually happened, we'd freeze up
trying to acquire an already-held lock.  Latest word is that this does
not explain Martin Pitt's trouble report, but it still looks like a bug.

Modified Files:
--------------
    pgsql/src/backend/access/nbtree:
        nbtutils.c (r1.82 -> r1.83)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtutils.c.diff?r1=1.82&r2=1.83)