When dentries are given an external name we currently allocate an entire qstr
for the external name.

This isn't needed.  We can use the internal qstr and kmalloc only the string
itself.  This saves 12 bytes from externally-allocated names and 4 bytes from
the dentry itself.

The saving of 4 bytes from the dentry doesn't actually decrease the dentry's
storage requirements, but it makes four more bytes available for internal
names, taking the internal/external ratio from 89% up to 93% on my 1.5M files.


Fix:

The qstr consolidation wasn't quite right, because it can cause qstr->len to
be unstable during lookup lockless traverasl.

Fix that up by taking d_lock earlier in lookup.  This serialises against
d_move.

Take the lock after comparing the parent and hash to preserve the
mostly-lockless behaviour.

This obsoletes d_movecount, which is removed.