Let default arc_c_max be platform dependent

Linux changed the default max ARC size to 1/2 of physical memory to
deal with shortcomings of the Linux SLUB allocator.  Other platforms
do not require the same logic.

Implement an arc_default_max() function to determine a default max ARC
size in platform code.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10155

module/os/linux/zfs/arc_os.c

@@ -60,6 +60,16 @@
 int64_t last_free_memory;
 free_memory_reason_t last_free_reason;
 
+/*
+ * Return a default max arc size based on the amount of physical memory.
+ */
+uint64_t
+arc_default_max(uint64_t min, uint64_t allmem)
+{
+	/* Default to 1/2 of all memory. */
+	return (MAX(allmem / 2, min));
+}
+
 #ifdef _KERNEL
 /*
  * Return maximum amount of memory that we could possibly use.  Reduced