arc_default_max on Linux should match FreeBSD

Commits 518b487 and 23bdb07 changed the default ARC size limit on
Linux systems to 1/2 of physical memory, which has become too
strict for modern systems with large amounts of RAM. This patch
changes the default limit to match that of FreeBSD, so ZFS may
have a unified value on both platforms.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Edmund Nadolski <edmund.nadolski@ixsystems.com>
Closes #15437