fs/nfs: Fix used uninitialized warn in nfs4_slot_seqid_in_use()

Fix the following warn:

fs/nfs/nfs4session.c: In function ‘nfs4_slot_seqid_in_use’:
fs/nfs/nfs4session.c:203:54: warning: ‘cur_seq’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  if (nfs4_slot_get_seqid(tbl, slotid, &cur_seq) == 0 &&
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
      cur_seq == seq_nr && test_bit(slotid, tbl->used_slots))
      ~~~~~~~~~~~~~~~~~

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>