fibs: restrict jail_attach(2) if process fibnum >= numfibs in the jail.

Reported by: olivier
Tested by: olivier
Reviewed by: kp, glebius
Differential Revision: https://reviews.freebsd.org/D38505
MFC after: 1 week

(cherry picked from commit 69e7d9b7e6b369b760e1f189af2e25587b56a102)