pgsql: Put a critical section around update of hash index metapage.

Log Message:
-----------
Put a critical section around update of hash index metapage. Per
discussion with Qingqing Zhou.

Modified Files:
--------------
pgsql/src/backend/access/hash:
hashpage.c (r1.49 -> r1.50)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hashpage.c.diff?r1=1.49&r2=1.50)