NFSD: Add a helper to decode state_protect4_a

Refactor for clarity.

Also, remove a stale comment. Commit ed941643 ("nfsd: implement
machine credential support for some operations") added support for
SP4_MACH_CRED, so state_protect_a is no longer completely ignored.

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