vmem: disable debug.vmem_check by default
It has a prohibitive performance impact when running real workloads.
Note this only affects kernels with DIAGNOSTIC.
Reviewed by: markj
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31784
(cherry picked from commit 6352bbf7be9b312979a4110342ff4e18eb0fcfa4)
Signed-off-by: Loic <loic.f@hardenedbsd.org>