NFS: Ensure that we update the sequence id under the slot table lock

Fix a callback slot table regression.

Fixes: e937ee71

 ("nfs: Only update callback sequnce id when CB_SEQUENCE success")
Cc: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>