pgsql: Code review for dtrace probes added (so far) to 8.4.

Log Message:
-----------
Code review for dtrace probes added (so far) to 8.4. Adjust placement of
some bufmgr probes, take out redundant and memory-leak-inducing path arguments
to smgr__md__read__done and smgr__md__write__done, fix bogus attempt to
recalculate space used in sort__done, clean up formatting in places where
I'm not sure pgindent will do a nice job by itself.

Modified Files:
--------------
pgsql/src/backend/access/transam:
xlog.c (r1.333 -> r1.334)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.333&r2=1.334)
pgsql/src/backend/storage/buffer:
bufmgr.c (r1.245 -> r1.246)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c?r1=1.245&r2=1.246)
pgsql/src/backend/storage/smgr:
md.c (r1.144 -> r1.145)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/smgr/md.c?r1=1.144&r2=1.145)
pgsql/src/backend/utils:
probes.d (r1.6 -> r1.7)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/probes.d?r1=1.6&r2=1.7)
pgsql/src/backend/utils/sort:
tuplesort.c (r1.89 -> r1.90)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/sort/tuplesort.c?r1=1.89&r2=1.90)