NFS: Don't corrupt the value of pg_bytes_written in nfs_do_recoalesce()

The value of mirror->pg_bytes_written should only be updated after a
successful attempt to flush out the requests on the list.

Fixes: a7d42ddb

 ("nfs: add mirroring support to pgio layer")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>