cifs: Move cifs_loose_read_iter() and cifs_file_write_iter() to file.c

Move cifs_loose_read_iter() and cifs_file_write_iter() to file.c so that
they are colocated with similar functions rather than being split with
cifsfs.c.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org