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>