Increase ZFS_OBJ_MTX_SZ further

Larger values have been associated with better concurrency and less
hanging of memory reclaim operations. So raise this number
further.

The array is allocated from vmem_alloc so we're not TOO concerned
about how big it gets.

include/sys/zfs_vfsops.h

@@ -112,7 +112,7 @@ typedef struct zfs_sb {
 	uint64_t	z_groupquota_obj;
 	uint64_t	z_replay_eof;	/* New end of file - replay only */
 	sa_attr_type_t	*z_attr_table;	/* SA attr mapping->id */
-#define	ZFS_OBJ_MTX_SZ	256
+#define	ZFS_OBJ_MTX_SZ	768
 	kmutex_t	*z_hold_mtx;	/* znode hold locks */
 } zfs_sb_t;