nfsd: make nfsd4_session->se_flags a bool

While this holds the flags from the CREATE_SESSION request, nothing
ever consults them. The only flag used is NFS4_SESSION_DEAD. Make it a
simple bool instead.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>