shm_alloc(): cleanup

[kib]

Summary

Consistently use the shorter name 'obj' for the new object.
Set OBJ_POSIXSHM flag outside if, it is set on both pathes.

Suggested by:   alc

[kib]

I think that unlocked assignment to swp/phys_priv is fine,the field is never changed after the initialization. I would even set the flag lockless, since the object is not yet visible, but I think the consistency is useful there, e.g. for sanitizers.

[markj]

sys/kern/uipc_shm.c

947–948

The assignment to shmfd->shm_object here isn't needed.

[kib]

Remove unneeded assignment.