exportfs: change connectable argument to bit flags

Convert the bool connectable arguemnt into a bit flags argument and
define the EXPORT_FS_CONNECTABLE flag as a requested property of the
file handle.

We are going to add a flag for requesting non-decodeable file handles.

Acked-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230502124817.3070545-2-amir73il@gmail.com>