selftests/fuse: test post-EOF page zeroing when a file is extended

Add a regression test for the bug where extending a file left the tail of
the old partial EOF page exposing stale mmap-dirtied data instead of zeros.

The test is a self-contained raw /dev/fuse server (no libfuse dependency)
that runs without writeback_cache and returns FOPEN_KEEP_CACHE, the
configuration in which the bug is visible.  Its backing data is always zero
in the hole, so any non-zero byte a read sees is stale page-cache data.
All offsets are relative to the runtime page size.  Four cases:

  - write_extend: pollute the post-EOF tail, extend past it by writing into
    a later page, and verify the tail reads back as zero;
  - ftruncate_extend: same, but extend via ftruncate();
  - fallocate_extend: same, but extend via fallocate() at the old EOF;
  - extend_into_eof_page_preserves_data: an extending write landing inside
    the old EOF page must not be clobbered by the zeroing.

Each case fails without the fix and passes with it.

Signed-off-by: Jimmy Zuber <jamz@amazon.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>