pgsql: Change _mdfd_segpath() to return paths by value

Change _mdfd_segpath() to return paths by value

This basically mirrors the changes done in the predecessor commit. While there
isn't currently a need to get these paths in critical sections, it seems a
shame to unnecessarily allocate memory in these paths now that relpath()
doesn't allocate anymore.

Discussion: https://postgr.es/m/xeri5mla4b5syjd5a25nok5iez2kr3bm26j2qn4u7okzof2bmf@kwdh2vf7npra

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ecbff4378beecb0b1d12fc758538005a69821db1

Modified Files
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src/backend/storage/smgr/md.c | 58 +++++++++++++++++++++++++---------------
src/tools/pgindent/typedefs.list | 1 +
2 files changed, 38 insertions(+), 21 deletions(-)