Add mutex_enter_nested() which maps to mutex_lock_nested()

Add mutex_enter_nested() which maps to mutex_lock_nested()

Also add support for the "name" parameter in mutex_init().  The name
allows for better diagnostics, namely in /proc/lock_stats when
lock debugging is enabled.  Nested mutexes are necessary to support
CONFIG_PROVE_LOCKING. ZoL can use mutex_enter_nested()'s "class" argument
to to convey the locking hierarchy.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #439