[PATCH] fix posix lock on NFS

NFS client prevents mandatory lock, but there is a flaw on it; Locks are
possibly left if the mode is changed while locking.

This permits unlocking even if the mandatory lock bits are set.

Signed-off-by: ASANO Masahiro <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/nfs/file.c

···
 		return -EINVAL;
 
 	/* No mandatory locks over NFS */
-	if ((inode->i_mode & (S_ISGID | S_IXGRP)) == S_ISGID)
+	if ((inode->i_mode & (S_ISGID | S_IXGRP)) == S_ISGID &&
+	    fl->fl_type != F_UNLCK)
 		return -ENOLCK;
 
 	if (IS_GETLK(cmd))