NFSv4: Fix an infinite looping problem with the nfs4_state_manager

Commit 76db6d95 (nfs41: add session setup
to the state manager) introduces an infinite loop possibility in the NFSv4
state manager. By first checking nfs4_has_session() before clearing the
NFS4CLNT_SESSION_SETUP flag, it allows for a situation where someone sets
that flag, but it never gets cleared, and so the state manager loops.

In fact commit c3fad1b1 (nfs41: add session
reset to state manager) causes this to happen every time we get a network
partition error.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Tested-by: Daniel J Blueman <daniel.blueman@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>