ksmbd: copy overlapped range within the same file

cifs.ko request to copy overlapped range within the same file.
ksmbd is using vfs_copy_file_range for this, vfs_copy_file_range() does not
allow overlapped copying within the same file.
This patch use do_splice_direct() if offset and length are overlapped.

Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>