Increase default arc_c_min

Increase default arc_c_min

Increase the default arc_c_min value to which whichever is larger,
either 32M or 1/32 of total system memory.  This is advantageous for
systems with more than 1G of memory where performance issues may
occur when the ARC is allowed to collapse below a minimum size.
At the same time we want to use the bare minimum value which is
still functional so the filesystem can be used in very low memory
environments.

Reviewed-by: Tim Chase <tim@chase2k.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6659