kill getname_statx_lookup_flags()

LOOKUP_EMPTY is ignored by the only remaining user, and without
that 'getname_' prefix makes no sense.

Remove LOOKUP_EMPTY part, rename to statx_lookup_flags() and make
static.  It most likely is _not_ statx() specific, either, but
that's the next step.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>