Merge patch series "Create and use APIs to centralise locking for directory ops."

NeilBrown <neilb@ownmail.net> says:

This series is the next part of my effort to change directory-op
locking to allow multiple concurrent ops in a directory.  Ultimately we
will (in my plan) lock the target dentry(s) rather than the whole
parent directory.

To help with changing the locking protocol, this series centralises
locking and lookup in some helpers.  The various helpers are introduced
and then used in the same patch - roughly one patch per helper though
with various exceptions.

I haven't introduced these helpers into the various filesystems that
Al's tree-in-dcache series is changing.  That series introduces and
uses similar helpers tuned to the specific needs of that set of
filesystems.  Ultimately all the helpers will use the same backends
which can then be adjusted when it is time to change the locking
protocol.

One change that deserves highlighting is in patch 13 where vfs_mkdir()
is changed to unlock the parent on failure, as well as the current
behaviour of dput()ing the dentry on failure.  Once this change is in
place, the final step of both create and an remove sequences only
requires the target dentry, not the parent.  So e.g.  end_creating() is
only given the dentry (which may be IS_ERR() after vfs_mkdir()).  This
helps establish the pattern that it is the dentry that is being locked
and unlocked (the lock is currently held on dentry->d_parent->d_inode,
but that can change).

* patches from https://patch.msgid.link/20251113002050.676694-1-neilb@ownmail.net:
  VFS: introduce end_creating_keep()
  VFS: change vfs_mkdir() to unlock on failure.
  ecryptfs: use new start_creating/start_removing APIs
  Add start_renaming_two_dentries()
  VFS/ovl/smb: introduce start_renaming_dentry()
  VFS/nfsd/ovl: introduce start_renaming() and end_renaming()
  VFS: add start_creating_killable() and start_removing_killable()
  VFS: introduce start_removing_dentry()
  smb/server: use end_removing_noperm for for target of smb2_create_link()
  VFS: introduce start_creating_noperm() and start_removing_noperm()
  VFS/nfsd/cachefiles/ovl: introduce start_removing() and end_removing()
  VFS/nfsd/cachefiles/ovl: add start_creating() and end_creating()
  VFS: tidy up do_unlinkat()
  VFS: introduce start_dirop() and end_dirop()
  debugfs: rename end_creating() to debugfs_end_creating()

Link: https://patch.msgid.link/20251113002050.676694-1-neilb@ownmail.net
Signed-off-by: Christian Brauner <brauner@kernel.org>