nfs_common: fix doc warning

Fix gcc W=1 warning:

fs/nfs_common/grace.c:91: warning: Function parameter or member 'net' not described in 'locks_in_grace'
Signed-off-by: ChenXiaoSong <chenxiaosong2@huawei.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfs_common/grace.c

@@ -82,6 +82,7 @@ __state_in_grace(struct net *net, bool open)
 
 /**
  * locks_in_grace
+ * @net: network namespace
  *
  * Lock managers call this function to determine when it is OK for them
  * to answer ordinary lock requests, and when they should accept only