pgsql: Make DROP INDEX lock the parent table before locking the index.

Log Message:
-----------
Make DROP INDEX lock the parent table before locking the index. This behavior
is necessary to avoid deadlock against ordinary queries, but we'd broken it
with recent changes that made the DROP machinery lock the index before
arriving at index_drop. Per intermittent buildfarm failures.

Modified Files:
--------------
pgsql/src/backend/commands:
tablecmds.c (r1.257 -> r1.258)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c?r1=1.257&r2=1.258)