Replace zf_rwlock with a mutex

The rwlock implementation on linux does not perform as well as mutexes.
We can realize a performance benefit by replacing the zf_rwlock with a
mutex.  Local microbenchmarks show ~50% improvement, and over NFS we see
~5% improvement on several of the ZFS Performance Tests cases,
especially randwrite and seq_write.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #9062

include/sys/dmu_zfetch.h

@@ -56,7 +56,7 @@ typedef struct zstream {
 } zstream_t;
 
 typedef struct zfetch {
-	krwlock_t	zf_rwlock;	/* protects zfetch structure */
+	kmutex_t	zf_lock;	/* protects zfetch structure */
 	list_t		zf_stream;	/* list of zstream_t's */
 	struct dnode	*zf_dnode;	/* dnode that owns this zfetch */
 } zfetch_t;