pgsql: Create a distinct wait event for POSIX DSM allocation.

Create a distinct wait event for POSIX DSM allocation.

Previously we displayed "DSMFillZeroWrite" while in posix_fallocate(),
because we shared the same wait event for "mmap" and "posix" DSM types.
Let's introduce a new wait event "DSMAllocate", to be more accurate.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20220711174518.yldckniicknsxgzl%40awork3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7bae3bbf62d63cdd49ae4ca4a851cef0cdbe6ab5

Modified Files
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doc/src/sgml/monitoring.sgml            | 5 +++++
src/backend/storage/ipc/dsm_impl.c      | 2 +-
src/backend/utils/activity/wait_event.c | 3 +++
src/include/utils/wait_event.h          | 1 +
4 files changed, 10 insertions(+), 1 deletion(-)