NLM: Clean up nlmsvc_grant_reply locking

 Slightly simpler logic here makes it more trivial to verify that the up's
 and down's are balanced here.  Break out an assignment from a conditional
 while we're at it.

 Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
 Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>