VFS: move dentry_create() from fs/open.c to fs/namei.c

VFS: move dentry_create() from fs/open.c to fs/namei.c

To prepare knfsd's helper dentry_create(), move it to namei.c so that it
can access static functions within.  Callers of dentry_create() can be
viewed as being mostly done with lookup, but still need to perform a few
final checks.  In order to use atomic_open() we want dentry_create() to
be able to access:

	- vfs_prepare_mode
	- may_o_create
	- atomic_open

.. all of which have static declarations.

Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
Link: https://patch.msgid.link/42deec53a50e1676e5501f8f1e17967d47b83681.1764259052.git.bcodding@hammerspace.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>