Pipe messages start out life on a queue on the inode, but when first
read they're moved to the filp's private pointer.  So it's possible for
a poll here to return null even though there's a partially read message
available.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>