On some old GNU/Linux systems, invoking posix_fallocate() with
offset=0 would sometimes cause already allocated bytes in the
data file to be overwritten.

Fix a correctness regression that was introduced in
commit 420798a8
by invoking posix_fallocate() in a safer way.
A similar change was made in MDEV-5746 earlier.

os_file_get_size(): Avoid changing the state of the file handle,
by invoking fstat() instead of lseek().

os_file_set_size(): Determine the current size of the file
by os_file_get_size(), and then extend the file from that point
onwards.