pgsql: Move memory barrier in UnlockBufHdr to before releasing the lock

Move memory barrier in UnlockBufHdr to before releasing the lock.

This bug appears to have been introduced late in the development of
48354581a4 ("Allow Pin/UnpinBuffer to operate in a lockfree
manner.").

Found while debugging a bug which turned out to be independent of the
commit mentioned above.

Backpatch: -

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/87a3023c60f4bab551441492652ef19371847fd3

Modified Files
--------------
src/include/storage/buf_internals.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

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