NFS: Fix an I/O request leakage in nfs_do_recoalesce

Whether we need to exit early, or just reprocess the list, we
must not lost track of the request which failed to get recoalesced.

Fixes: 03d5eb65

 ("NFS: Fix a memory leak in nfs_do_recoalesce")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: stable@vger.kernel.org # v4.0+