When shmfs_truncate is called, it uses shmfs_alloc_object to create the
physical backing for the shm file. However, the allocated physical
memory returned by mm_pgalloc is not cleared when CONFIG_BUILD_KERNEL is
set, which is a clear POSIX violation:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/truncate.html
"If the file was previously shorter than length, its size is increased,
and the extended area appears as if it were zero-filled."
For FLAT and PROTECTED modes zalloc is used, so the violation affects
KERNEL mode only.
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