Increase ZFS_OBJ_MTX_SZ to 256

Increasing this limit costs us 6144 bytes of memory per mounted
filesystem, but this is small price to pay for accomplishing
the following:

* Allows for up to 256-way concurreny when performing lookups
  which helps performance when there are a large number of
  processes.

* Minimizes the likelyhood of encountering the deadlock
  described in issue #1101.  Because vmalloc() won't strictly
  honor __GFP_FS there is still a very remote chance of a
  deadlock.  See the zfsonlinux/spl@043f9b57 commit.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1101

include/sys/zfs_vfsops.h

@@ -87,7 +87,7 @@ 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	64
+#define	ZFS_OBJ_MTX_SZ	256
 	kmutex_t	z_hold_mtx[ZFS_OBJ_MTX_SZ];	/* znode hold locks */
 } zfs_sb_t;