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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: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>