sysctl kern.geom.journal.cache.limit shows negative value for FreeBSD/amd64
system having over 4GB RAM. That's due to:

1. the limit being u_int instead of u_long like vm.kmem_size (the limit is half of vm.kmem_size by default for amd64);
2. sysctl handler g_journal_cache_limit_sysctl() using u_int instead of u_long.

The fix is to replace u_int with u_long for the kern.geom.journal.cache.limit
sysctl variable.

PR: 198500
Submitted by: Dr. Andreas Longwitz <longwitz@incore.de>
Reported by: Eugene Grosbein
Discussed with: kib
MFC after: 1 week