The sysctl provides the listing on named linked posix shared memory
segments existing in the system.

Reuse shm_fill_kinfo() for filling individual struct kinfo_file.
Remove unneeded lock around reading of shmfd->shm_mode.

Reviewed by:	jilles, tmunro
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D20258