NFS: move the deleg_cur check out of nfs_detach_delegation_locked

NFS: move the deleg_cur check out of nfs_detach_delegation_locked

nfs_inode_set_delegation as the only direct caller of
nfs_detach_delegation_locked already check this under cl_lock, so
don't repeat it.

Replace the lockdep coverage for the lock that was implicitly provided by
the rcu_dereference_protected call that is removed with an explicit
lockdep assert to keep the coverage.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>