NFS: Fix WARN_ON due to unionization of nfs_inode.nrequests

Fixes the following WARN_ON
WARNING: CPU: 2 PID: 18678 at fs/nfs/inode.c:123 nfs_clear_inode+0x3b/0x50 [nfs]
...
Call Trace:
  nfs4_evict_inode+0x57/0x70 [nfsv4]
  evict+0xd1/0x180
  dispose_list+0x48/0x60
  evict_inodes+0x156/0x190
  generic_shutdown_super+0x37/0x110
  nfs_kill_super+0x1d/0x40 [nfs]
  deactivate_locked_super+0x36/0xa0

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>