subvol of locks has to be access-control, not storag/posix:

volume posix2
    type storage/posix
#   option o-direct enable # (default: disable) boolean type only
#   option export-statfs-size no # (default: yes) boolean type only
#   option mandate-attribute off # (default: on) boolean type only
#   option span-devices 8 # (default: 0) integer value
#   option background-unlink yes # (default: no) boolean type
    option directory /export/home/dir2
end-volume

volume posix-ac2
    type features/access-control
    subvolumes posix2
end-volume

volume brick2
    type features/locks
#   option mandatory on # Default off, used in specific applications
    subvolumes posix2
end-volume