pgsql: Rearrange lazy-vacuum code a little bit to reduce the window

Log Message:
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Rearrange lazy-vacuum code a little bit to reduce the window between
truncating the table and transaction commit. This isn't really making
it safe, but at least there is no good reason to do free space map
cleanup within the risk window. Don't lock out cancel interrupts
until we have to, either.

Modified Files:
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pgsql/src/backend/commands:
vacuumlazy.c (r1.129 -> r1.130)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c?r1=1.129&r2=1.130)