[regression] braino in "lustre: use is_root_inode()"

In one of the places (ll_md_blocking_ast()) we had open-coded
!is_root_inode(inode) and replaced it with is_root_inode(inode).
See the last chunk of f76c23:
-                   inode != inode->i_sb->s_root->d_inode)
+                   is_root_inode(inode))
should've been
+                   !is_root_inode(inode))
obviously...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>