pgsql: Avoid SIGBUS on Linux when a DSM memory request overruns tmpfs.

Avoid SIGBUS on Linux when a DSM memory request overruns tmpfs.

On Linux, shared memory segments created with shm_open() are backed by
swap files created in tmpfs. If the swap file needs to be extended,
but there's no tmpfs space left, you get a very unfriendly SIGBUS trap.
To avoid this, force allocation of the full request size when we create
the segment. This adds a few cycles, but none that we wouldn't expend
later anyway, assuming the request isn't hugely bigger than the actual
need.

Make this code #ifdef __linux__, because (a) there's not currently a
reason to think the same problem exists on other platforms, and (b)
applying posix_fallocate() to an FD created by shm_open() isn't very
portable anyway.

Back-patch to 9.4 where the DSM code came in.

Thomas Munro, per a bug report from Amul Sul

Discussion: https://postgr.es/m/1002664500.12301802.1471008223422.JavaMail.yahoo@mail.yahoo.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/899bd785c0edf376077d3f5d65c316f92c1b64b5

Modified Files
--------------
configure | 2 +-
configure.in | 2 +-
src/backend/storage/ipc/dsm_impl.c | 54 ++++++++++++++++++++++++++++++++++++--
src/include/pg_config.h.in | 3 +++
src/include/pg_config.h.win32 | 3 +++
5 files changed, 60 insertions(+), 4 deletions(-)