pgsql: Rethink PROCSIGNAL_BARRIER_SMGRRELEASE.

Rethink PROCSIGNAL_BARRIER_SMGRRELEASE.

With sufficiently bad luck, it was possible for IssuePendingWritebacks()
to reopen a file after we'd processed PROCSIGNAL_BARRIER_SMGRRELEASE and
before the file was unlinked by some other backend. That left a small
hole in commit 4eb21763's plan to fix all spurious errors from DROP
TABLESPACE and similar on Windows.

Fix by closing md.c's segments, instead of just closing fd.c's
descriptors, and then teaching smgrwriteback() not to open files that
aren't already open.

Reported-by: Andres Freund <andres(at)anarazel(dot)de>
Reviewed-by: Robert Haas <robertmhaas(at)gmail(dot)com>
Discussion: https://postgr.es/m/20220209220004.kb3dgtn2x2k2gtdm%40alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b74e94dc27fdbb13954f230b1d1298430afa6c0c

Modified Files
--------------
src/backend/storage/smgr/md.c | 22 +++++++++++---------
src/backend/storage/smgr/smgr.c | 45 +++++++++++++++++++++++++++++++++--------
src/include/storage/md.h | 1 -
src/include/storage/smgr.h | 2 ++
4 files changed, 52 insertions(+), 18 deletions(-)