selinux: Use simple_start_creating() / simple_done_creating()

selinux: Use simple_start_creating() / simple_done_creating()

Instead of explicitly locking the parent and performing a lookup in
selinux, use simple_start_creating(), and then use
simple_done_creating() to unlock.

This extends the region that the directory is locked for, and also
performs a lookup.
The lock extension is of no real consequence.
The lookup uses simple_lookup() and so always succeeds.  Thus when
d_make_persistent() is called the dentry will already be hashed.
d_make_persistent() handles this case.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20260224222542.3458677-7-neilb@ownmail.net
Signed-off-by: Christian Brauner <brauner@kernel.org>