fuse: fix copy_file_range() in the writeback case

Prior to sending COPY_FILE_RANGE to userspace filesystem, we must flush all
dirty pages in both the source and destination files.

This patch adds the missing flush of the source file.

Tested on libfuse-3.5.0 with:

  libfuse/example/passthrough_ll /mnt/fuse/ -o writeback
  libfuse/test/test_syscalls /mnt/fuse/tmp/test

Fixes: 88bc7d50

 ("fuse: add support for copy_file_range()")
Cc: <stable@vger.kernel.org> # v4.20
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>