nfsd: use fsnotify group lock helpers

Before commit 9542e6a6

 ("nfsd: Containerise filecache laundrette")
nfsd would close open files in direct reclaim context and that could
cause a deadlock when fsnotify mark allocation went into direct reclaim
and nfsd shrinker tried to free existing fsnotify marks.

To avoid issues like this in future code, set the FSNOTIFY_GROUP_NOFS
flag on nfsd fsnotify group to prevent going into direct reclaim from
fsnotify_add_inode_mark().

Link: https://lore.kernel.org/r/20220422120327.3459282-10-amir73il@gmail.com
Suggested-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220321112310.vpr7oxro2xkz5llh@quack3.lan/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>