Prevent races in accesses of the software crypto session array.

swcr_newsession can change the pointer for swcr_sessions which races with
swcr_process which is looking up entries in this array.

Add a rwlock that protects changes to the array pointer so that
swcr_newsession and swcr_process no longer race.

Original patch by:	Steve O'Hara-Smith <Steve.OHaraSmith@isilon.com>
Reviewed by:		jmg
Sponsored by:		EMC / Isilon Storage Division
Fix clang warning.