pgsql: Fix mdsyncfiletag(), take II.

Fix mdsyncfiletag(), take II.

The previous commit failed to consider that FileGetRawDesc() might
not return a valid fd, as discovered on the build farm.  Switch to
using the File interface only.

Back-patch to 12, like the previous commit.

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fd005e1a87446ad61a6c950841b0a6f93bdfb594

Modified Files
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src/backend/storage/smgr/md.c | 25 +++++++++----------------
1 file changed, 9 insertions(+), 16 deletions(-)