NFS: Fix list manipulation snafus in fs/nfs/direct.c

Fix 2 bugs in nfs_direct_write_reschedule:

 - The request needs to be removed from the 'reqs' list before it can
   be added to 'failed'.
 - Fix an infinite loop if the 'failed' list is non-empty.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>