pgsql: Fix two bugs in setting the vm bit of empty pages.

Fix two bugs in setting the vm bit of empty pages.

Use a critical section when setting the all-visible flag on an empty page,
and WAL-logging it. log_newpage_buffer() contains an assertion that it
must be called inside a critical section, and it's the right thing to do
when modifying a buffer anyway.

Also, the page should be marked dirty before calling log_newpage_buffer(),
per the comment in log_newpage_buffer() and src/backend/access/transam/README.

Patch by Andres Freund, in response to my report. Backpatch to 9.2, like
the patch that introduced these bugs (a6370fd9).

Branch
------
REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/4da24f12e63313b7dbb6b3e3d0317e04045df636

Modified Files
--------------
src/backend/commands/vacuumlazy.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)