nfs4_op_delegreturn should hold the state_lock while using found state

After finding a delegation state, the current nfs4_op_delegreturn() drops
cache inode entry's read-write lock, state_lock, and still uses
found state. This commit uses state_unlock_locked and avoid dropping the
lock!

Signed-off-by: Malahal Naineni <malahal@us.ibm.com>