fs: touch up predicts in path lookup

fs: touch up predicts in path lookup

Rationale:
- ND_ROOT_PRESET is only set in a condition already marked unlikely
- LOOKUP_IS_SCOPED already has unlikely on it, but inconsistently
  applied
- set_root() only fails if there is a bug
- most names are not empty (see !*s)
- most of the time path_init() does not encounter LOOKUP_CACHED without
  LOOKUP_RCU
- LOOKUP_IN_ROOT is a rarely seen flag

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251105150630.756606-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>