NFSD: Protect against filesystem freezing

Flole observes this WARNING on occasion:

[1210423.486503] WARNING: CPU: 8 PID: 1524732 at fs/ext4/ext4_jbd2.c:75 ext4_journal_check_start+0x68/0xb0

Reported-by:  <flole@flole.de>
Suggested-by: Jan Kara <jack@suse.cz>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=217123
Fixes: 73da852e

 ("nfsd: use vfs_iter_read/write")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>