os_file_set_size(): Let us invoke the Linux system call fallocate(2)
directly, because the GNU libc posix_fallocate() implements a fallback
that writes to the file 1 byte every 4096 or fewer bytes. In one
environment, invoking fallocate() directly would lead to 4 times the
file growth rate during ALTER TABLE. Presumably, what happened was
that the NFS server used a smaller allocation block size than 4096 bytes
and therefore created a heavily fragmented sparse file when
posix_fallocate() was used. For example, extending a file by 4 MiB
would create 1,024 file fragments. When the file is actually being
written to with data, it would be "unsparsed".

The built-in EOPNOTSUPP fallback in os_file_set_size() writes a buffer
of 1 MiB of NUL bytes. This was always used on musl libc and other
Linux implementations of posix_fallocate().