vfs: document locking requirements for d_move, __d_move and d_materialise_unique
Adding a comment to d_materialise_unique per Al's request...
d_move and __d_move have some pretty substantial locking requirements,
but they are not clearly documented. Add some comments spelling them
out. Also, document the requirement for the i_mutex of the parent in
d_materialise_unique.
Cc: Al Viro <viro@ZenIV.linux.org.uk> Signed-off-by: Jeff Layton <jlayton@redhat.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>