posixshm: Fix a double unlock in shm_partial_page_invalidate()

[markj]

Summary

For some reason, shm_partial_page_invalidate() unlocks the object upon
an error, but its callers don't expect this. Don't do any special error
handling. Keep the subroutine anyway since the name is a bit clearer
than vm_page_grab_zero_partial().

While here, normalize the object pointer used for locking in
shm_deallocate().

Fixes: 454bc887f250 ("uipc_shm: Implements fspacectl(2) support")