We currently insert socket dentries into the global dentry hashtable.
This is *suboptimal* because there is currently no way these entries can be 
used for a lookup(). (/proc/xxx/fd/xxx uses a different mechanism). Inserting 
them in dentry hashtable slow dcache lookups.


To let __dpath() still work correctly (ie not adding a " (deleted)") after 
dentry name, we do : 

- Right after d_alloc(), pretend they are hashed by clearing the 
DCACHE_UNHASHED bit. 

- Call d_instantiate() instead of d_add() : dentry is not inserted in hash 
table.

__dpath() & friends work as intended during dentry lifetime.

- At dismantle time, once dput() must clear the dentry, setting again 
DCACHE_UNHASHED bit inside the custom d_delete() function provided by socket 
code, so that dput() can just kill_it.

Signed-off-by: Eric Dumazet <redacted>