When reading an existing FSM or VM page that was found to be corrupt by the
buffer manager, the code applied PageInit() to reinitialize the page, but
did so without any locking.  There is thus a hazard that two backends might
concurrently do PageInit, which in itself would still be OK, but the slower
one might then zero over subsequent data changes applied by the faster one.
Even that is unlikely to be fatal; but it's not desirable, so add locking
to prevent it.

This does not add any locking overhead in the normal code path where the
page is OK.  It's not immediately obvious that that's safe, but I believe
it is, for reasons explained in the added comments.

Problem noted by R P Asim.  It's been like this for a long time, so
back-patch to all supported branches.

Discussion: https://postgr.es/m/CANXE4Te4G0TGq6cr0-TvwP0H4BNiK_-hB5gHe8mF+nz0mcYfMQ@mail.gmail.com