Do not collapse objects with OBJ_NOSPLIT backing swap object.

NOSPLIT swap objects are not anonymous, they are used by tmpfs regular
files and POSIX shared memory. For such objects, collapse is not
permitted.

Reported by: mjg
Reviewed by: markj, trasz
Tested by: mjg, pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D19923