Remove rwlock wrappers

The only remaining consumer of the rwlock compatibility wrappers
is ztest.  Remove the wrappers and convert the few remaining
calls to the underlying pthread functions.

    rwlock_init()    -> pthread_rwlock_init()
    rwlock_destroy() -> pthread_rwlock_destroy()
    rw_rdlock()      -> pthread_rwlock_rdlock()
    rw_wrlock()      -> pthread_rwlock_wrlock()
    rw_unlock()      -> pthread_rwlock_unlock()

Note pthread_rwlock_init() defaults to PTHREAD_PROCESS_PRIVATE
which is equivilant to the USYNC_THREAD behavior.  There is no
functional change.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7591

include/sys/zfs_context.h

@@ -86,7 +86,6 @@
 #include <strings.h>
 #include <pthread.h>
 #include <setjmp.h>
-#include <synch.h>
 #include <assert.h>
 #include <alloca.h>
 #include <umem.h>