Unpin buffer before inplace update waits for an XID to end.

Unpin buffer before inplace update waits for an XID to end.

Commit a07e03fd8fa7daf4d1356f7cb501ffe784ea6257 changed inplace updates
to wait for heap_update() commands like GRANT TABLE and GRANT DATABASE.
By keeping the pin during that wait, a sequence of autovacuum workers
and an uncommitted GRANT starved one foreground LockBufferForCleanup()
for six minutes, on buildfarm member sarus.  Prevent, at the cost of a
bit of complexity.  Back-patch to v12, like the earlier commit.  That
commit and heap_inplace_lock() have not yet appeared in any release.

Discussion: https://postgr.es/m/20241026184936.ae.nmisch@google.com