shm_alloc(): cleanup
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
Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D47018