NFS: Clean up nfs41_check_expired_stateid()

Clean up: Instead of open-coded flag manipulation, use test_bit() and
clear_bit() just like all other accessors of the state->flag field.
This also eliminates several unnecessary implicit integer type
conversions.

To make it absolutely clear what is going on, a number of comments
are introduced.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>