NFS: Fix missing pg_cleanup after nfs_pageio_cond_complete()

Commit a7d42ddb ("nfs: add mirroring
support to pgio layer") moved pg_cleanup out of the path when there was
non-sequental I/O that needed to be flushed.  The result is that for
layouts that have more than one layout segment per file, the pg_lseg is not
cleared, so we can end up hitting the WARN_ON_ONCE(req_start >= seg_end) in
pnfs_generic_pg_test since the pg_lseg will be pointing to that
previously-flushed layout segment.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Fixes: a7d42ddb ("nfs: add mirroring support to pgio layer")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>