FreeBSD: Remove accidental ARC size limiter

i386 has some additional memory reservation logic that limits the size
of the reported available memory.  This was accidentally being used on
all arches due to a missing header.

Include machine/vmparam.h in freebsd/zfs/arc_os.c to pull in the
missing UMA_MD_SMALL_ALLOC definition.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10616

module/os/freebsd/zfs/arc_os.c

@@ -46,6 +46,7 @@
 #include <sys/aggsum.h>
 #include <sys/vnode.h>
 #include <cityhash.h>
+#include <machine/vmparam.h>
 
 extern struct vfsops zfs_vfsops;