vfs: Let prison_enforce_statfs zero the fsid

Currently, we unconditionally zero the fsid before returning a struct
statfs to a jailed process. Move this into prison_enforce_statfs() so
it only happens if enforce_statfs is greater than 1, or enforce_statfs
is 1 but the mountpoint is outside the jail.

PR: 291301
MFC after: 1 week
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D54214

(cherry picked from commit d4f25d0c7957f0f1960028eec82625c2d6405537)