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.

Too big and you get memory allocation failures. This number (767)
was chosen semi-arbitrarily but 3000+ has been shown to be too high.

include/sys/zfs_vfsops.h

@@ -90,7 +90,7 @@ 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	767
 	kmutex_t	z_hold_mtx[ZFS_OBJ_MTX_SZ];	/* znode hold locks */
 } zfs_sb_t;