Update arc_available_memory() to check freemem

While Linux doesn't provide detailed information about the state of
the VM it does provide us total free pages.  This information should
be incorporated in to the arc_available_memory() calculation rather
than solely relying on a signal from direct reclaim.  Conceptually
this brings arc_available_memory() back in sync with illumos.

It is also desirable that the target amount of free memory be tunable
on a system.  While the default values are expected to work well
for most workloads there may be cases where custom values are needed.
The zfs_arc_sys_free module option was added for this purpose.

zfs_arc_sys_free - The target number of bytes the ARC should leave
                   as free memory on the system.  This value can
                   checked in /proc/spl/kstat/zfs/arcstats and
                   setting this module option will override the
                   default value.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3637

man/man5/zfs-module-parameters.5

@@ -566,6 +566,19 @@ log2(fraction of arc to reclaim)
 Default value: \fB5\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_arc_sys_free\fR (ulong)
+.ad
+.RS 12n
+The target number of bytes the ARC should leave as free memory on the system.
+Defaults to the larger of 1/64 of physical memory or 512K.  Setting this
+option to a non-zero value will override the default.
+.sp
+Default value: \fB0\fR.
+.RE
+
 .sp
 .ne 2
 .na