NFSD: Clear wcc data between compound ops

Testing NFS4.0 by pynfs, I got some messeages as,
"nfsd: inode locked twice during operation."

When one compound RPC contains two or more ops that locks
the filehandle,the second op will cause the message.

As two SETATTR ops, after the first SETATTR, nfsd will not call
fh_put() to release current filehandle, it means filehandle have
unlocked with fh_post_saved = 1.
The second SETATTR find fh_post_saved = 1, and printk the message.

v2: introduce helper fh_clear_wcc().

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>