nfscl: Add a "has acquired a delegation" flag for delegations

A problem was reported via email, where a large (130000+) accumulation
of NFSv4 opens on an NFSv4 mount caused significant lock contention
on the mutex used to protect the client mount's open/lock state.
Although the root cause for the accumulation of opens was not
resolved, it is obvious that the NFSv4 client is not designed to
handle 100000+ opens efficiently.

For a common case where delegations are not being issued by the
NFSv4 server, the code acquires the mutex lock for open/lock state,
finds the delegation list empty and just unlocks the mutex and returns.
This patch adds an NFS mount point flag that is set when a delegation
is issued for the mount. Then the patched code checks for this flag
before acquiring the open/lock mutex, avoiding the need to acquire
the lock for the case where delegations are not being issued by the
NFSv4 server.
This change appears to be performance neutral for a small number
of opens, but should reduce lock contention for a large number of opens
for the common case where server is not issuing delegations.

This commit should not affect the high level semantics of delegation
handling.

MFC after: 2 weeks