MFC uipc_shm: Fix kern.ipc.posix_shm_list for jails

Fix error return of kern.ipc.posix_shm_list, which caused it (and thus
"posixshmcontrol ls") to fail for all jails that didn't happen to own
the last shm object in the list.

(cherry picked from commit 747a47261eee59b6e9c437cd2c1b3979df5c32ac)