nfs/vfs: discard d_exact_alias()

nfs/vfs: discard d_exact_alias()

d_exact_alias() is a descendent of d_add_unique() which was introduced
20 years ago mostly likely to work around problems with NFS servers of
the time.  It is now not used in several situations were it was
originally needed and there have been no reports of problems -
presumably the old NFS servers have been improved.  This only place it
is now use is in NFSv4 code and the old problematic servers are thought
to have been v2/v3 only.

There is no clear benefit in reusing a unhashed() dentry which happens
to have the same name as the dentry we are adding.

So this patch removes d_exact_alias() and the one place that it is used.

Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20250226062135.2043651-2-neilb@suse.de
Signed-off-by: Christian Brauner <brauner@kernel.org>