NFSv4.1: Replace dprintk() in pnfs_update_layout with something less buggy

Dereferencing nfsi->layout in order to read plh_flags without holding
a spin lock is bug prone. Furthermore, the dprintk() tells you nothing
about whether or not the call succeeded.
Replace it with something that tells you about whether or not a valid
layout segment was returned for the inode in question.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>