NFSv4: Further clean-ups of delegation stateid validation

Change the name to reflect what we're really doing: testing two
stateids for whether or not they match according the the rules in
RFC3530 and RFC5661.
Move the code from callback_proc.c to nfs4proc.c

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>